There are no NFL players who could play rugby. They are already established within the sport (which incidentally uses a similar shaped ball, and is a contact sport (but that's where the similarity ends)).

When they've reached the NFL they are trained and conditioned for extremely high impact, short stint action.(spot the oxygen bottles on the touchline in case an OL guy has to run more than 50ft)

As for those who could have become rugby players instead of NFL players, then I'd say 99.99% of them could have. (Please note: It's 'Could have' not 'Could of'(pet hate of mine))

Why? Because some are huge, and some are extremely quick guys. Some are too big to do anything that requires stamina, but I'd imagine they wouldn't have become quite as big if the sport they were playing required stamina.