If I use about:profiles for my profiles, can I add an avatar?

If you want to migrate, you can just copy the contents of the profile folder from the original to an empty newly created profile in the modern system. Please be mindful of the differences though (like the new system either launches the recently used profile, or always asks — but there's no way to launch a fixed specific profile every single time etc.)

The original profiles have nowhere to display any avatars, so they don't have that functionality.