I have a Mec-Gar magazine made for the cZ75 B 9mm.It however is only 10 round capacity.Can I put a plus 2 floorplate extension on it.
You don't have a picture, so it's harder to tell. Some 10 round magazines have indentions in the side of the metal body that keep the follower from moving below that point - that's what limits the magazine to 10 rounds. Some of them have a shorter metal body with a big plastic base on them that limits the downward travel of the follower.
If you live in a state that allows more than 10 round magazines it's not a bad idea to order new ones.