Adding Avatar Skeleton to Non-FBX

I've made a character for which I will need several Mecanim layers, and thus I need several masks. The problem is that while the character is originally built in Maya, I have made radical changes to it in Unity since then. In order to make a working mask, I therefore need the skeleton of the in-game character, not the FBX. Does anyone know how to add an avatar skeleton to a Unity object, or any other solution?

 

That's the problem. The character has no skeleton mask, so I need to know how to add one for it, if possible.

 

I believe we are misunderstanding each other, so I'll try to clarify my problem.
I have already created an avatar mask like this one.

The problem is that the character I'm trying to create the mask for doesn't have anything to fill this box. It's constructed in Unity with different shapes, and I can't make it in a 3D modeling program. So I need to create one of these:

But I don't know how to do that without using a 3D modeling program, which as stated I can't do for this character.

 

I actually ended up giving up on my old model and made a new one, so everything works now. But still, these are some really useful links to have if I ever find myself in this situation again. So thanks!