Putting in a detachable mag may be illegal, depending on a lot of stuff. If it never had a detachable mag and it's in stock configuration, it would be illegal to put a detachable mag in it.
If you're using this for hunting, per the forum you chose to ask it in, one magazine full should be enough for a hunting trip. If it's not enough, you either really stink or are trying to kill too many animals at once. Your state will likely have a magazine capacity limit. My state does not, but it does not allow semi-auto rifles for hunting either.