You can do what you want to but when you bulge out or split then end of a barrel don't complain. Like I said it won't happen immediately but if you shoot a lot of steel shot it ups your chances signifigantly of this. It will also give you a pressure spike just as the steel enters the choke tube.
I guess all the manufacturers put warnings "LEAD SHOT ONLY" just because?
If you don't believe me give Briley a call and ask them. Or take a look through Brownells and read the descriptions of all the choke tubes. A short, rapidly constricting choke tube, like what comes with a Mossberg 500, is not designed to shoot steel unless they have been specifically hardened for steel shot. And even then you can only shoot steel with certain constrictions.
If steel shot bulged barrels with chokes made for lead, there would be hundreds of thousands of older (pre-screw-in-choke) shotguns out there with bulged barrels from shooting steel shot through them, and this isn't the case.
About three weeks ago I cut about 6 inches off an old double barrel gun that the owner shot steel shot through and split the ends of one of the barrels. Now he doesn't have any choke at all on that gun.
I am not trying to sound like a jerk here I just want to make it clear that it is not a good idea to shoot steel through a choke not designed for it.