To resize and add a border/background with a predefined color you could use the free BorderFX plug-in, see: http://www.iborderfx.com/borderfx,(don't use the link on Aperture's plug-in page - that points to an incompatible version) use the plug-in as an Export plug-in and specify the output size you want. Set background and border to the same color. Save the parameters a an export preset BorderFX. If you select multiple images in Aperture, BorderFX will batch export them with the same settings.
It cannot be done with the tools provided by Aperture, but the plug-in works fine, for example: