You need a factory extended barrel. Not common at all.
Mark McWillis of TROS does the backthreading you are talking about. People used to pay stupid money for a factory extended length P9S barrel until he developed that technique, but if you wanted a .45 P9S set up for a can, a factory barrel (or at least a new over length threaded barrel installed on a factory trunnion) is your only option, there is not enough "meat" to do the work safely.
I found a factory P7 barrel that had been previously cut/threaded 1/2x28". Sent it to H&K USA for installation. Got it back, took it to the range, shot it, listed it for sale. Bluntly put, P7 series (unless we are talking K3 in .22LR or .32, 9mmk the can is too big) suck for suppressing. Looks cool as all get out, but loud. LOUD. That pop when the gas system vents is atrocious.
Save your money.