There has been alot of rumours about REX2 now being an open file format, open source etc.
None of this is true!
The fact is that Propellerhead Software have made it possible for any company to apply for a REX2 Import license. We have done so and received the sdk.
It is not source code explaining the rex2 file format though, it's simply a dll (a program) that we/BeatCreator/etc can use/call to import rex2 files. There is no information about how to export to rex2 and according to Propellerhead's web site they do not plan to release any such information either (unfortunately!).
However, we do plan to add rex2 import to BeatCreator, BeatCreator Pro, TrackCreator and BeatSlicer. But as we believe it's rex2 export that you long for, we don't currently put rex2 import at the top of our priority list. Still, if you feel that rex2 import is what's missing in BeatCreator, why not send us a mail and we might change our priorities (note we have so many new cool features on the list and so little time).