If you've been playing the harmonica for a while and can do draw/blow bends with ease, then go ahead and start working on overblows! If you're a new player, then I don't think overblowing would be the easiest technique to learn at this stage.
Overblowing won't damage your harmonica as long as you do it right. Don't be misled by the term "overblow". Contrary to what many people think, you don't have to blow or draw hard to get an overbent note. It's just a matter of proper technique and finesse. You can play overbends as easily as normal bends if your harmonica is properly set up.