This is a federal law, not Californian one.

You can do a blind pin where you drift a pin in which can only go in one way to lock the suppressor onto the barrel. Most people drill the pin all the way though and then silver solder the holes in each direction.

The other way is to use silver solder to weld the thing into place. You need to know what you're doing. You clean the barrel and suppressor completely, heat the work up using a mapp torch (propane doesn't get hot enough) and then run soldier onto the threads while keeping the work hot enough to tread the suppressor the rest of the way over the hot liquid metal.

A cheap enough job for a gunsmith. Federal authorities have been known to arrest people for the mear possession of illegal parts - assembled or not. I always buy my 14.5" barrels pre-legalized with the suppressor attached. I'd recommend that you do that in the future but wouldn't lose any sleep if I were you now.