Such a thing could never exist, because the point of tokumx is to change the storage system, so at some point you have to change the storage over and that's just going to be a rewrite of all your data. It sucks but that's the way it is.
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.
