Yes you can shoot slugs out of that gun. It's not going to hurt it in any way. You can also shoot slugs out of an O/U just fine as well as long as it hasn't been back bored and still is the standard .729" diameter bore. Although I have never seen anyone actually do this out of an O/U.

You are not going to find a rifled barrel for that gun so you will need to use rifled slugs or forester type slugs and not saboted slugs. I suggest an improved cylinder choke.