Sure, you can just add it AFAIK. You have to be careful adding addons though. They can defeat your security by phoning home, and blowing your cover. You also have to make sure NoScript is set to block everything. It's inexplicably left open by default, and is a huge security risk.
As I understand it, Adblock+ should be safe to add to the Tor browser, but you should confirm that elsewhere, and look at the source code yourself. Even if you don't code, many times things that look wrong will jump out at you. FWIW, I don't use addons other than default in Tor.