Actually, you can do that. I wrote a tool to allow replication from MongoDB to TokuMX: http://www.tokutek.com/2013/07/tokumx-1-0-3-seamless-migrati.... It doesn't allow TokuMX instances to satisfy write concern or to participate in elections, but you can slave a TokuMX replica set off a MongoDB replica set just fine, and this allows you to do a rolling migration where you replace one secondary at a time.