An adapter won't work. The nipple needs to be properly seated in the threaded inlet at the proper depth for the nipple to work properly. You will probably need to take the fill nipple pin out and match the existing oring. Go to an oring store and get help if needed. If the oring is not there, find one that fits the pin properly. Make sure it is tight and is a high durometer rating or it will get distorted and blow off under high pressure.
Except for the Automag in front, its usually the man behind the equipment that counts.