I think you're out of luck. Nobody that I know makes aftermarket threaded barrels for Rugers. Being sort of a budget gun (no offense, they work fine), there probably isn't a market for much in the way of aftermarket accessories.
If you really want a suppressed .45, you're probably going to have to go with another platform.
Most the ones I've seen have been:hmm...that's a damn shame. What platforms would I need to be looking at for a suppressed .45?
I can't find anything on the subject, nor anybody who makes custom barrels for the p345...any idea's for adding a suppressor?