You can't add accent marks in GarageBand. You could do that (and a whole lot more) using dedicated notation software like Finale or Sibelius.
However, you can change the loudness of a single MIDI note or any region in Garageband in other ways.
For MIDI, open up the editor (the scissor icon), select the note and change the velocity.
For a region of any kind, you can create a volume curve (type "track automation" or "volume curve" in the help menu for more info on how to do this). Using volume curves, you can make the volume ramp up or down very quickly, or gradually, for any part of your song. By zooming in, you can increase the precision.
No need to buy an expansion pack!