PRNav capability can be gained by installing something like a 430W installed as an IFR fit. You don't even need to have auto slewing HSI's. In an N reg, you are now PRNav approved in the USA.
To get PRNav approval in Europe you need a LOA issued by a FSDO in the USA, which will normally be issued by bringing your aircraft documents in and showing the FMS and test results (all checklists supplied by Garmin with a 430W). I can see no mention of crew training or anything else like that, and AFAIAA that is all that would need to be done to be PRNav approved.
In a G reg you'd probably have to have pink underpants, and have a funny handshake though....