Do many (or any?) players transfer into the MLS from USL sides? Is it similar to, say, playing for Wellington Olympic and then moving to the Phoenix?
It's rare, but it does happen. Usually the players that do move up tend to go into MLS Next Pro. As you would expect, a lot of loan deals from MLS sides to USL clubs.
Think the league record outbound transfer from the USL Championship was set this year - $1,000,000 fee from an Egyptian Premier League outfit.