Hey Andrew. I was able to be directed to the solution by another user on another thread.

Essentially the steps are as follows:

1-Open 3DXchange and drag the file you exported from Rokoko Studio into it. (I always use the Maya-Human - FBX Export setting.)
2- Click the convert to non standard.
3- Under the presets, I chose the Maya Human Ik
4-You should get the green confirmation dot indicating its all good. RATHER THAN CONVERT, at the top, CLICK SAVE next to CHARACTERIZATION Profile and name the profile whatever you want "MyCustomRigThingy"
5- Save in :  C:\Program Files\Reallusion\iClone 8\Program\Assets\Share\CharaterizeProfiles6

Now when you drag the same animation you dropped into 3DXchange, it should let you choose "MyCustomRigThingy" from the drop down and should work. Let me know if it does.