Assuming you are dragging & dropping MIDI clips for your basic grooves/fills, it might be easier to
> turn on smart grid mode
> expand the MIDI drum track to show takes lanes
> click on the [+] to add a new take lane
> drag & drop a fill pattern at the desired point
> slip edit the overlapping portion of the original groove clip to allow room for the fill clip
This will keep all MIDI clips on one track for Session Drummer and you can turn off Smart Grid mode and slide the clips around as desired. You can also select a clip in the take lane and switch to the PRV which will show the selected notes. They can be dragged as a group in the PRV if you find it easier to position.