It's been quite a while but 15 years ago or so some one on here converted one. IIRC it involved chopping the stock right behind the receiver & and scabbing on a folder.
IMO it ruined a good gun & destroyed an irreplaceable stock. Folding stocks may not be around forever & once a PC9 is cut up you can't undo.
Unlike other guns that honking big counter weight in the forearm pretty much eliminates other options.