Based on a conversation with my gunsmith about a year of so ago. I have a custom commercial FN Mauser chambered to the 7x57. This no serial number for some odd reason and the action used did not appear to have a number. According to my gunsmith, the only way I could legally put a serial number on the gun was take to to the BATF and have them assign a number.
Best answer I really can give is call the nearest BATF office and ask but I'm pretty much positive that you can't legally put a serial number yourself.
PJ

Our forefathers did not politely protest the British.They did not vote them out of office, nor did they impeach the king,march on the capitol or ask permission for their rights. ----------------They just shot them.
MOLON LABE