At the beginning of Act III where Meredith is arguing with Orsino about the future of Kirkwall you get at some point a middle choice "I will rule". Maybe picking it leads to Hawke taking the crown for himself? I wouldn't know because I opted to side with the mages.
Lengyel, magyar ket jo barat, egyutt harcol, s issza borat. -- Hungarians FTW! XBL/PSN gamertag: Poganin
I believe it's in act 3. When you get the letter from Alistair(or whoever the ruler of Ferelden is for you)and you go to talk to them, there's an option for becoming the Viscount there I'm pretty sure. I didn't try it on my first playthrough and I'm only in act 2 on my second so I'm not 100% sure about that.