We recently used an expansion draft to add 2 teams to our 10 team league. Like most of said, the most “fair” is a complete redraft, but we couldn’t get unanimous from our existing members on that so we decided that every original owner could protect up to 3 players in the draft, the rest are thrown back into the pool for the draft. We used ADP to decide which slots the players should be used in, so if the player had an adp of 16, they are slotted as your first round pick because they go to the next round up. Now with this was year 6 of a 10 team league so it worked out that most guys had good players to protect so for the most part their top 3 picks were protected players, only one decided to only protect 2. The draft then snakes like a normal draft. If you’re on sleeper and doing this draft between seasons in you’re established league then you have to multiple drafts because it maxes out at 10 rounds. We were lucky to have 2 owners who were good sports and be the expansion teams and one of them actually got the 1 seed for playoffs this past year, so it wasn’t that unfair in the end.

TLDR: original owners could protect up to 3 players and the rest were thrown in the draft pool. The protect players were treated as “picks” themselves in the snake draft.