Actually, since iPhoto and Aperture now share the same library, adding the photo to iPhoto by definition makes the photo available under Aperture as well.
But only, if you have opened the Aperture Library as the current iPhoto Library. If you are using iPhot and aperture on different libraries, to be able to compare two photo libraries opened at the same time, you are out of luck.
In Mail, there is a Save pull down menu. Did Mavericks add Aperture?
I don't see these menus any more in Mail.app in Mavericks, only the contextual menu I have shown in the screenshot. To save an attachment, I ctrl-click the attachment. And then I can "Export to iPhoto", but not to the current Aperture library - unless it happens to be the iPhoto library as well, as Rex Ross pointed out.