I don't know how hard this would be to add to the code, but an idea that popped into my head is this:
Create the ability to take grenades from the grenade throwing enemies (like how most enemies you knife them when you're in close proximity to them) instead of knifing them the Alien would grab and take a grenade which could be throw with the "d" key, providing
player_grenades == true
just a thought :)
what would also be funny is you could maybe blow yourself up if you're not careful ;)