I think nymphing a winter steelhead rig on a 5 wt will be a challenge. I'm not a nympher, but if I were to try it I'd fish an ESL like D3 said and a glow bug or bead or Lifter or Caballero, etc. as a dropper.

For ease of casting a nymphing rig I set up a line last year that I think will better serve the purpose. For my 8 wt rod I took a WF10F line and cut off the front 8' and formed a loop. The front taper doesn't turn over the split shot or weighted nymph so well, so removing it and letting the heavier belly section do the heavy work seems pretty functional. I just need to put some time into it and see how well it works out.

Sg