You could get a 9mm Barrel from Ruger for your 40 gun and simply have a gunsmith ream it. No other modifications should be necessary to have a 357 Sig gun with a spare 40 barrel. I'm fairly sure that Ruger doesn't sell Barrels alone but I've never tried. Hypothetically, you could acquire both a 9mm and .40 gun and then send the 9mm to Ruger sans barrel and have them 'fit' a new barrel. When the gun comes back, sell the 9mm P94. In the meantime, give the 9mm barrel and 40 gun to a gunsmith for a quick reaming. XXX out the 9mm and stamp .357 Sig on the barrel and you're set.
Disclaimer: I am not a lawyer, warranty specialist, or anybody you should trust. Do not attempt anything I'm saying here without a dose of YOUR gunsmith's advice. Having said that, I'd do it in a heartbeat.
Note: Ruger doesn't HAVE a warranty to void!