If you are referring to the tracking-to-avatar mapping method, yes, custom models can use the free-form retargeting method, such as most of FaceRig's models, although there are no technical specifications or guidelines available on how to prepare the models, right now. I cannot estimate when will this documentation be ready.

If you are referring to mapping the movements of the avatar from FaceRig to another model outside the app (such as to Max, Maya, MotionBuilder, etc), this feature is available only for the Studio version of the app, which allows the user to export avatar movement as animation (.fbx or .dae file).

I've answered the two cases, due to the term's (retargeting) ambiguity. Hope this answers your question.