You do not need a rifled barrel. You can shoot the rifled slugs (sometimes called Foster-style slugs) out of your smoothbore. Use the most open choke you have, usually IC.

Buckshot will shoot fine out of your smoothbore as well. Try different chokes to see which one patterns the best. And, be sure to check the hunting regulations where you live. Not all states are buckshot-friendly for deer hunting.

If you want to use the newer sabot-type slugs, then you will need a rifled choke tube, at a minimum, or better yet, a fully-rifled barrel. I don't shoot slugs, so I cannot tell you first hand what kind of performance you will get with a rifled choke tube, but I have heard good things from those shooting sabots through fully-rifled barrels.