You can't use a straight tube, but you can with some care use two straight drills to drill the required (tapered) hole. If you drill on the lathe it will be fairly easy. If on a drill press, you need a way to drill both holes without moving the pen blank. IIRC, it is more of a stepped hole than a tapered hole.
First drill the entire blank to the smaller diameter. Next, mark the larger diameter drill with a piece of masking tape to the depth you want to drill, and enlarge the smaller hole to that depth. It isn't terrible critical that the depth of the large diameter matches the step in the tube, as long as there is sufficient legth of material left in the blank to support the small end of the tube.
