Lomu could have been a star given his pace and ferocity - wouldn't be especially big by NFL standards nowadays but 6'4, 18 stone and that pace 25 years ago (100m in 10.7, no idea of his 40m times), stood out like a sore thumb.
No idea if he'd have had the mental capacity to learn playbooks and the nuances of NFL runs though -
a lot more complicated than Union (which I love). I'd also question whether a lot of Union players would have the desire/mental strength to move to USA... imagine some of the Irish/Welsh/Southern Islanders in US? Can't see it.
Martin Offiah may have made a decent WR?
Tough ask nowadays - RBs usually shorter, stocky but have pace. Most similar shape in Union is prop forward and they have 'limited' pace?
Wingers to WRs is a possibility, 2nd row/back row to line (Off or Def) maybe.
Out of current players, Mario Itoje? His ability in rucks and mauls is amazing (one of the best forwards in world rugby), quick on his feet, intelligent which would help with decision making and only 25. Maybe play in the defensive line (defensive end?)... 6'4, 18 stone, mobile, great timing (which you have to have playing lock else you get pinged by ref all the time in rugby).