Is it possible? During our last game a frined of mine tried to do this all the time and I couldn't find a satisfying explanation why this shouldn't be possible (except for balancing issues). It does make sense and if I'm correct, it isn't addressed anywhere directly in the rules (at least not the german rules). So... Any clear answer on that?

On page 16 in the premium rules:

"Unoccupied areas may never be the target of attack"

Furthermore combat involves always an enemy area, whereto unoccpied areas do not belong.

So I think, unoccupied areas may only in the movement phase be "conquered".