I believe this is not possible now. It would be very useful to be able to tag a generic character as an avatar. In my case I use generic characters because I have a blinking animation and I didn't wanted to get into the live2d madness just for that. I use the avatar as the player in first person, not as a representation of the characters talking, maybe other people has a similar issue.
I see. Well, avatars can only be textures and they should be of a specific size (resolution). If you just assign regular character there, it will be severely resized and look bad. I'd suggest making special textures for the avatars (by cropping and resizing the original character textures); those textures can then be used with any character implementations.