Quite honestly i try to prevent crossovers. But then i currently only GM RT; since the movers & shakers aspect is really to my liking, as well as the sandbox mode.
In RT you can copy close to any class from DH. Well mostly. My preference is to simply give the player in question his pick on the origin path, recreating the DH's characters history as closely as possible. Yes he will end up less powerful, but he will get access to the PF in the end.
DW is more of a problem, but then FFG has invented Astartes type bolters all of the sudden; and i simply dont want Explorators or cybered up/tricked out Arch-Militants running around with Astartes type weaponry. Plus i have 10 years of 40k in me, Having different bolters all of the sudden is really, really strange (well strange enough to be ignored/ especially since it contradicts 40k itself).