Most gunsmiths will do the job (for around $75-100 I think but am not 100% sure so I'd recommend checking around). Add 20 bucks per choke tube on top of this. I'd want a good smith for the job as if the threads are not cut square, you could possibly end up with a pattern that is off to one side. Cutting down the barrel would run extra, maybe $40-50. You are almost as far ahead to look at an aftermarket or replacement barrel, most of the new ones will already have the threads and would likely be cheaper in the long run. For not all that much more, maybe 75-100 bucks, you could just get a new shotgun like an 870 Express and let's face it who could not use an extra piece in the cabinet?
Safe shooting.