You're talking RP-wise only? I'd say go for it.
There are still quite a few races not accounted for in the Combine universe (you can get a feel for some of them by visiting the Race Suggestion forum
). Many of those are near-human, and don't offer enough variation from current near-human races to be included.
Others haven't got enough info or graphics (as in the current Stenax race suggestion) to make them viable as player-races.
But as far as simply RP-ing a mixed-race character? i'd say go for it, as long as you allow your DM to have final say-so over the other race's characteristics, and how they are used within that RP.
Fr example, before the Diathim were approved, I doubt there'd have been many DMs who would have allowed a PC to go flying through space without any gear or hindrances. But now that they are an accepted race, there are many people who RP them for their rather unique skills, one of which is their flying capability.
I'd love to hear more about which race you're referring to, and the story behind the hook up between your parents.