yes, it is.
It's similar to the method how the helmet is added on the human head?
wtfbbq? no, the helmetless-head and head-with-helmet models have no such relation to eachother.
If I make it so i dont need to write any shader file?
if you only want to add outlines, then all you need to use is 1 small shader for a monotonic (black) color.
How will the performance be wenn i do this with every Model?
i suspect that there will only be a minor performance degradation.