You can add new females just as you add new fish to any tank... just acclimate them first. If you want, you could put them in a breeding trap for a few days so all the fish can get used to seeing one-another before they actually meet face-to-face, but in my experience, it doesn't make that big a difference. When adding new female bettas, one of two things can happen:
A ) The new female will challenge the alpha for dominance, and they will fight.
B ) The new female will immediately back down from the alpha, thus the alpha retains her dominance without a struggle.
It's just the luck of the draw which you get. You also need to be aware that some females are just as aggressive as males, and so may need to be housed alone or with fish that don't draw their aggression. You just need to keep a close eye on them for a few days and see how everything works out. I find it helpful to add new females at feeding time, so by the time they finish eating, they're confused as to how someone else suddenly appeared in the tank
I added my last two females this way, and have not seen even one torn fin.
Edit: lol, added spaces because my "B )" made a B)