No it cannot be done online.
If you want a FAA Commercial certificate then you have to complete the FAA CPL Written exam and pass the CPL Checkride.
Your foreign hours will count but you must make sure that you have all the Aeronautical experience documented in the FAR 61.127 and 61.129.
Also - as this would be your first issue of a FAA certificate - you will need to get TSA clearance - regardless whether you come to the US or not.
You cannot get a FAA CPL validation based on the strength of holding a Foreign certificate (FAR 61.75).
You can ONLY get a Private certificate issued under FAR 61.75 based on a foreign certificate and you still have to visit the FSDO or there is a guy in the UK who can do it for you (at a cost).
If you only want to get a FAR 61.75 Private cert - you will have to check if the PC-12 aircraft insurance will permit it to be flown by a PPL.