There are no female astartes and there never will be and the fluff is absolutely concrete that the made up science doesn't work on the lady people with the boob things. That's as far as it goes; if you introduce real science 40k falls apart at the best of times. The only thing I can think of that's even a vague shot is the Daemonculabula, which was a. a woman with astartes gene seed and b. as far as I know accepted female 'aspirants' as well as male, but what crawled out after 'the process' was usually hard to call a space marine at the best of times. God knows what it'd do to someone that wasn't already an ideal astartes candidate i.e. a woman.