The only thing I can think of is that you might be doing something wrong in Excel, but can't figure out what. I just spent over an hour messing with it and it always works for me. I even imported 39 legends from the hidden M25 roster into the newest official roster and no problems at all.
If anyone wants that roster, I uploaded it to community files as LEGENDSWITHPORTRAITS, gamertag The Zig. I might edit them to all be rookies and upload that as well.
Be warned, though, that I only imported the ones that had portraits. There were quite a few (including some of my favorite players) for whom the portraits did not show up. They either cleaned out some of the portraits or somehow the IDs don't match up anymore.