I asked PCGS USA this once upon a time, because i had some freebie vouchers for grading that PCGSEUROPE wouldn't take, they could only be used if you sent directly to America yourself, the answer was incredibly complicated and intricate but it is possible but seemed so much of a ball ache i didn't bother. Luckily enough i now have the option of NGC grading here in Munich which cuts out around €100 in postage fees alone.......
If you have a PCGS or NGC office or dealer who uses either of those 2 companies they will do all that legwork for you as i have sent coins to PCGS Paris which in turn were forwarded to America for grading there