You can add notes above/below existing notes. You can add multiple notes at the same time, according to their intervals relative to the existing notes.
Prerequisite
If you want to use the mouse, you have enabled mouse editing.
Procedure
Result
Notes are added to the selected notes according to the intervals specified.
Tip
You can also add notes above/below existing notes by Alt/Opt-dragging selected notes upwards/downwards.
When you drag notes, shadow noteheads indicate their destination pitches, and the affected region appears highlighted with the rhythmic grid shown above it.