I've played a little doumbek, but don't consider myself a "player" - and after looking at some of those videos, I'm even more humbled ;-) But it's clear to me that you need strength in a part of the hand that normally isn't well-developed - the big knuckle in the hands. Most of our hand strength comes from muscles way back in the forearms, but the velocity of the strike in those finger rolls appears to come from them being extended and moved from the big knuckle.
So you might take a look at an exercise that I use to help my guitar students develop their "bar chords", which require a lot of strength and independence in that part of the hand:
http://music.stackexchange.com/a/4747/951
I'll be interested to hear if it helps - good luck!