An 870 bbl can be threaded for tubes by a gunsmith, provided your bbl is thick enough. They have 'thinwall' tubes made by colonial.
FWIW... I had one threaded and had a full choke installed. It worked fine until a buddy of mine stuffed a high brass #4 in it and blew the choke right out, ruining the bbl.
So, yes. It can be done, but I'd stick to standard or low powered loads.