Is it possible to use the brushes at runtime?

Hey guys, I'm making a RTS game and I'm trying to implement a "blight" area (like the Undead race in Warcraft 3). I managed to do t$$anonymous$$s using the SetAlphamaps function. The problem is that I can only draw square shapes. So, does anybody know if it is possible to paint the terrain using the brushes at runtime?

Sorry for my english, not my native language.