Just recently did just that to a 500cc 3K aluminum bottle. Fitted to an old school THEOBEN RAPID that had to have bottle removed to fill otherwise. ( what a PITA )Found the bottom thickness once threaded 1/8-27 NPT has as many or more threads than the Foster fill fitting to be used.
Man, that's one butiful rifle
My Theoben bottle that has the Allen Z fill port on it is just fine. The hole that goes through the bottle is threaded, though there is also an aluminum nut inside the bottle that will pinch the fill adapter to the bottle. There are no issues with this method, as it has been done many times for many years.