Surely it would be.

If you've got the common event set up to change the appearance based on a key.

And you could probably use variables and the Key processing thingy?

If at the start of the game you have a tutorial, and say that you can change your character appearance by pressing a key? Then say

"To become Yado press A" and store the key pressed as a variable.
"If you want to become Derth Vadar press S" and store it as a variable again?

I'm just hazarding here, so don't take anything as written.