I found a rather old topic on this and am hoping there is some positive news. I am new to CC3, so I may not understand what is going on, but I got it with the understanding that I could import fbx clothing from other sources, and now it appears that fbx models only import if they have a key, which so far, none of the models I have found have. Does this mean I can only use Reallusion models? This is a serious problem for me. Besides wanting to use a variety of models from the web, I was planning on creating my own in Blender, which exports FBX. Does anyone know how ordinary FBX files can be used in CC3? If they can't be, the product is much, much less useful than I thought when I bought it.

It's not clear from the menu, but you want to use "Create Accessory" which does not require a key.  Then just reweight it as cloth.

---

Alienware Aurora R16, Win 11, i9-149000KF, 3.20GHz CPU, 64GB RAM, RTX 4090 (24GB), Samsung 870 Pro 8TB, Gen3 MVNe M-2 SSD, 4TBx2, 39" Alienware Widescreen Monitor
Mike "ex-genius" Kelley

Thanks! That works! It looks like fitting random stuff on the net may be a nightmare, but if I build my own stuff in Blender on the base model, that should work great. Thanks for your prompt response.
The manual doesn't seem to address this, and you're right, it's not clear from the menu at all. Good thing this forum exists.

Just more FYI, but I find that most clothes and hair from the net in general IS a PITA to get fitted.

Daz stuff, OTOH, brought in via Transformer is a piece of cake and in some respects even better than stuff from the marketplace or RL themselves.

---

Alienware Aurora R16, Win 11, i9-149000KF, 3.20GHz CPU, 64GB RAM, RTX 4090 (24GB), Samsung 870 Pro 8TB, Gen3 MVNe M-2 SSD, 4TBx2, 39" Alienware Widescreen Monitor
Mike "ex-genius" Kelley

The Create Accessory option works great, as long as you are not trying to put any of your Props, Cloths, and or hair on the Content Store, and or Content Market Place. If you are just needing it, and or wanting it for your own projects then Create Asset with obj's works great. 

I have brought in my own Swords, Staffs, Axes, Clothing, and such. I haven't done hair yet, but I am still learning the Hair Tool for Blender. 

Here is a few shots of things I have done. Most of the clothing and props are mine, and some are reallusion content to finish the character.

The FBX key is needed as you found to transfer models back into CC3 and iClone as an FBX file. Where you get the key from is ( You export out a base character in the T pose as an FBX ) this will make an FBX Key. in the file where your exported character is you will see a file folder right beneath it that says FBX Key.
Now for example: go into CC3. When the initial project is loaded, it is the cc.base female in the A pose. So go into poses on the left content screen, in template and look for the motions folder. Find the T pose for the female. The icon will show a female in greyscale (in other words the little picture of the female in the thumbnail will be grey with no texture.) Click on this to pose the default cc.female base character in the scene into the T pose. 
Now go up to the file button. Select Export as an FBX. It will give you another box with options in the middle of the screen to include motions and textures and such. That is not important, just go ahead and hit OK. 
Select to where to export her to, and or pay attention to where its exporting her to.
Now go into file explorer and there should be the FBX Female that you exported, and you could import her into Blender / 3DS Max, ect. There should be the FBX key right beneath the actual FBX object. 
Now if you import her into Blender (or the program of choice) as an FBX she should be in there as a Grey Shape of the female base and her Rigging bones should be visible also.
If you want to export her back out with IE lets say a Blouse that you made, of in my case a Sword, then you Have or want to Scale the object to proportion to her. A Blouse you would want to fit to her body so its not clipping and is in the T pose.

The tricky part is now. You have to attach the Blouse / IE object to the Avatar bone hierarchy. I still dont exactly know how to do this. I had a guy on Skype from Germany show me once, and I should have had my screen recorder on at the time. I think he will walk me through it again some time, but he has a plateful with the Corona crap at the time.
Anyway if you can attach the clothing/Sword/hair to the avatar, then you Export it from Blender as either the same file name, or you can change it. I think it is just easier to use the same file name wich overwrites the base FBX with the same one, but with the added piece/s.

Now go back into CC3, and choose import from the file menu, (not the Create button) Choose import FBX. It will give you the box with the FBX slot to ad the key. Right click and copy the FBX Key that was generated from the initial export that I described earlier. 
Paste the FBX key into the slot and hit import, or OK in the box, and it will bring the cc.female_base back into CC3 with the models that you attached to her. 
You will most likely have to add textures separate. That is a whole other ball of wax. Anyway I hope that helps. I know haw it works now, but like I said there are still steps that I dont know how to do.

Haratio! Thank you so much! This is exactly the kind of info I need. I'm new at Blender, too, but I think if you Add Single Bone to the sword, for example, you can then make it a child of the figure's hand, or hip, or whatever. The question is, in CC, can it be taken off, or is it now part of the figure? Also, not sure how it would work with clothing, because I haven't got that far in Blender yet.  Either way, I would certainly like to be able to separate the prop from the figure in CC3 and be able to use it with other figures. I hope there is a different kind of 'attach' in Blender that is meant- like being in the same file, or being grouped when exported, or something.
You make excellent props and clothing, btw. I hope I'm able to get to that level before I die of old age!

Good to know. 

I noticed that bringing in a piece of clothing via Create Accessory (ie, without an fbx key) causes the 'conform' button to be greyed out, which makes the fitting more arduous than it should be. Do you know of a work-around? Do you know why this is? It seems like conforming would just be a matter of cycling through the vertices along the normal or anti-normal until they are on the correct side of the figure's boundary, so I don't understand why it's greyed out.  If I follow Haratio's scheme and, say, just put a shirt more or less on an exported CC3 avatar, will the conform button work when the come back into CC3?