Is it possible to create an avatar for a pre-existing character that was made in unity?

I am mostly a programmer, and I recently came across a problem in which stunted the progress of a game I am working on. I created my character in unity, and created a series of animations based around a rig I made in unity, which was simply a heirarchy of empty game objects which served as pivots for the rig. Since I had not attempted this before, I wasn't sure whether or not I could use the mecanim system for the prefab I had made, and after scrolling the internet and looking in unity itself I couldn't find a way to create an avatar without using a file imported from an external source.

Is there any way that I can apply the Avatar component to a pre-existing prefab that is rigged correctly, or will I have to keep using the old legacy system?

Thanks in advance, Adam