Preface with a few things:
I am pretty new to FL as well, but I think i can give ou good advice.
There is probabaly a better forum somewhere, comp. rec forum or something, but no matter.
You use the, what'sitcalled, playlist, to put up your chorus, verse parts right? Add a new pattern for the fill and add jsut that part, you might need to remove the bar that is playing the regular part, but then just add the bar for the new pattern in it's place.
As far as timing, drum fills will happen within the same framework as the rest of the bars of the music. In most standard 4 beat-per-measure rock music the fills will occur at the end of a bar leading (from the "3" or "4") into the "1" of the next measure. That is put very simply-- obviously fills can be placed as creatively as you can imagine. Listen closely to a similiar fill and count out the measures and you will see how the beats tend to lay out in the measure.
Hope this is somewhat helpful.
Daav