I know two guys who did this, but they were SO's for 2 1/2 years and a PCA of some kind is also required.
Joe
I spoke with an S/O who just did this as well. Taking a base in the U.S. somewhere in May. You have to serve a year as a S/O. He said that his upgrade interview for Freighter F/O was right after his one year anniversary. He is going from the Bus to the 747 for what it is worth. He also said you need to meet FO minimums which it seems like most S/Os I fly with these days already do before they were hired as S/Os.