There's a long standing bias with mainly us old greybeard old school gamers ..call it a taboo, superstition, sense of impropriety, stubborn individuality or whatever... many of us refuse to play if forced to use pre-generated characters. This includes games like 40k. We don't want to field no stinkin' "UltraMarines" or "Space Wolves" or any sort of Angels no matter how you dress them up. We definitely don't want no swatty Nancy-Marine "Rainbow Warriors"....
Luckily, some of the GW design team were of the same mindset, which is why there were originally 1000 undefined Space Marine Chapters. As the "canon" grew, these 1000 "undefined" Chapters got "defined" but the powers that be left us two loopholes. God bless 'em for not wishing to stifle our creativity!
There exists (or may not exist!) two Space Marine Legions (1st Founding Chapters) that have officially been expunged from all Imperial records. Even knowing about them is Heresy most foul. Nobody knows their names, organization, Primarchs, nothing...it's not even sure if the records were purged before or after the Horus Heresy. Nobody knows why they were expunged either, whether they still exist somewhere out there...nothing.
Of course I'm leading up to the justification for the possible existence of female Space Marines...
I realize, it would be difficult not to break canon by including them into a "normal" game of Deathwatch. Female or not, the missing Legions would never be in a sanctioned Deathwatch team as even the most powerful, fringest of the fringe Inquisitors would probably have never heard of such a thing as 2 mysterious unknown Space Marine Legions and would certainly know that such knowledge was abominable Heresy.
However, I'm merely pointing out that if you felt so inclined you could base a Campaign around a Chapter, female of course, of these mythical Astartes which operate outside Imperial society, either in hiding or literally outside Imperial space where they've been fighting (insert bad guy of choice) for the last 30-40,000 years without any Imperial contact.