Not a good idea...
If you're playing in a tight-knit league...say with 12 members in an ESPN league that has been active since 2006...I imagine the Commissioner would rather fill that opening with somebody that was known personally by at least one person in the league. This league...which probably keeps records back to 2006...would lose some of its charm if a member were brought in just from a message board. In fact, I bet the Commissioner of the league would rather just drop 2 teams and redraft the dynasty.