It is ok to shoot slugs out of your field barrel with a modified or improved cylinder choke tube. NEVER shoot anything with the tube completely removed. I forget the manufacturer,but there are adjustable sights available that clamp on vent ribs.
If it were me, I'd look for a used 870 or Mossberg 500 with a deticated slug barrel rather than try finding a slug barrel for the Stoeger. They come through HK/Benelli now and dealers are scarce. (at least here) I haven't priced Stoeger barrels for a long time, but I do remember them being rather high.
If you're looking for this season, look NOW. Used deer guns start going fast this time of year, and are almost impossible to find as season gets closer. Just like I tell everyone I know, look for these kinda things in the opposite season....deer guns in summer, varmint rifles in fall. The used racks are full of them when the season is opposite.