There is no option to transfer Skin weights when I import an item as an accessory. This is a custom character I imported and rigged within CC4. I need to attach the hair.  

Transfer skin weights only works for CC characters. For custom Humanoid characters, you will need to weight in an external application.

I replied to the similar question here: https://forum.reallusion.com/FindPost525351.aspx

But I would like to elaborate a bit after further testing.
Re-rigging with AccuRig works fine consuming and weighting accessories.

One feature which does not work well in this workflow, is extrapolating guides from skeleton.
It looks like it does not really extrapolate, but rather creates a new set of guides anyhow.
So if you tweaked guides while rigging your character originally, you have to tweak guides again while re-rigging.

Here is an example.
On the left is Heidi character which I rigged with AccuRig and tweaked clavicle guides to fix slouchy shoulders in a process.
On the right is the same character after I rigged it again to weight Hair Accessory which supposed to extrapolate guides from the existing skeleton.
That did not happen. Shoulders as slouchy as if a new set of guides was created. That is bug I think and it needs to be fixed!




On the other hand, the hair should remain an accessory IMHO anyway and be weighted 100% to the head. Then create a proper physics weight map and position collision shapes.
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There is also another way to move an accessory mesh under character hierarchy.
By simply saving characterization profile, exporting to FBX (Mesh and Expression Sequence), importing that FBX back specifying saved profile in characterization.
Then restore face blend-shapes (if any existed) as sequence from the same FBX file.
The Accessory mesh would be moved under characters' meshes hierarchy, but still remain an "accessory" and would have its own bone.
On the other hand you can now Edit Weights for that accessory and weight it custom to any other armature bone.

Thanks I'll test it.