I bought a plastic "skin fitting" from a boat place and it actually filled the hole in the lid nicely and left a decent length of half inch thread hanging out the top, but I couldn't get it too seal. I then went back to the drilled hole in the side idea for the tire valve and that worked OK.
BUT - I think the idea above of 2 x stainless washers and a brass nipple sounds great. Seal the top and end up with a decent length of half inch thread hanging out the top to add what ever you need.
My isuse is I've added the tire valve and that is sealed well, the top is sealed, but where I attach the beer line to the original fitting I get a slow leak. Be OK if you were filling from a keg for a party as the leak is slow, but I'd like to condition in the plastic "keg" so needs to be fully sealed.
C & B - Richard.