Got Photoshop ? Set up Aperture preferences to use Photoshop as external editor. Export in PSD format. Add text layer, type, save and you'll find it back in Aperture complete with layers so you can re-edit if needed.
Check out guygowan.com for great Aperture/Photoshop tutorials (subscription needed). Worth it to learn a LOT !!
I'm nothing to do with him. I just rate his approach.