You can add apps (the .ipa) to the iTunes library just by copying them over. But the apps are linked to your iTunes account, and whether or not you have purchased them. Don't know why you would want to do this, because its only use as I can see it, is to move an app from one iTunes library to another (from upto a max of 5 authorised libraries).
As for hacked apps, not allowed here so you best keep those questions to yourself.