Sounds like a pretty good lifepath, though what you pick for your duty will probably determine why exactly you ended up washing out of the Space Marines.  As far as inserting your character goes, he could either be a stowaway on a Deathwatch ship who ended up in Port Wander, or he could have been a stowaway on the<em>Reliquary of Alexander</em>the entire time, and perhaps comes to the crew with proof that their propaganda about Sarah being the scion of an ultramarine is a pack of lies.