simple answer
* Go to the bottom drum machine (I think it's the 909?)
* Select the snare drum.
* On the pattern editor, click on every beat.
Hey presto ... 16 semiquavers. That's all a drum roll is.
You might want to double up some of the beats (i.e. make them into two demisemiquavers). To do this, I think you have to click on a button to the left of the pattern editor (it goes red or green I think) then click on the beats you want to double. You would probably only want to do this to the 8th and/or 16th beats. You might have to change the speed of the 'roll' so that you only get two hits instead of 3 or more.
To get a build up, export a few bars of the snare roll to a WAV file and use a wave editor to fade it in.