most "sporting" style guns...won't shoot high enough to float a bird for Trap...so buying a "Trap" barrel may not be what you really want to do...
But if you want something like an unsingle ...give Joel Etchen guns a call ..and talk to them / see what they can do for you ..../ but the barrel will have to be custom fit to the receiver ....and an extra barrel for an O/U is probably going to cost you at least $ 1,500...
so if you really want a combo gun...you might consider trading the gun you have to someone like Etchen...and buying something else.../ but most of us want a gun that handles, feels different, shoots higher, longer barrels for Trap than we do for Sporting Clays, Skeet, 5 Stand or hunting...and we usually turn to stand alone guns for Trap specifically ( in 32" and around 10lbs ) vs more of a general gun in 30" and around 8 lbs for sporting, skeet,etc....
But you can't just buy a barrel set and expect it to fit the receiver either....they may come close to locking up properly / but close isn't good.