If it's a true 5rd mag...then it will be very close to flush with the mag well when it's inserted into the weapon and as such...it won't matter what kind of follower you put in it.
You'd need to completely chop off the legs of the follower to even come close to getting 10 to fit.
If you can see enough camo sticking out the bottom that you like the pattern, I'm guessing it's a blocked 10 rounder already.
Take the plate off the bottom, pull out the spring and follower, and see if there is anything else inside it.
Take some pics while you're at it. It will make this an easy fix if it's possible or completely squash it if it's not.