They already do this.
One of the MAIN features of FaceRig is that you can import your own characters and models, as long as you know how to use blender and rigging.
For example, this is a tutorial for importing props:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=309911042
There is also documentation included with the program for this. (<steam folder>/common/facerig/bin/docs)
Avatars are already possible and done too, for example here is a minecraft Steve character:
http://facerig.us/index.php?resources/minecraft-steve-avatar.16/
(This site is community made/managed and should not be confused with https://facerig.com which is managed by Holotech Studio's, the creators of FaceRig.)

ok
sorry