my Astropath wants to get a force weapon, the rules in DH for them was made more for psykers from that game so how can I work it for an astropath, should I make him manifest the extra hit as a psycker in DH would?

The astropath in our game uses a force weapon. We just treat the opposed Willpower test as the psychic test (like many other RT powers) and ignore the threshold bit. It's working fine.