Lengthening the MIDI clip should lengthen the playback time of the whole project.
Hover your mouse over the right-hand edge of the clip until the mouse cursor changes to a square with an arrow in it, then hold the left button down while dragging to the right. This will stretch the clip without actually altering any MIDI data, and you should be able to add any amount of silence you want.
If this doesn't work - and I've never seen it not work - you can always insert a dummy MIDI event via the PRV or the Event List.