You need an external editor to do that. Take a look at Pixelmator in the Mac App Store. It was on sale for $15. I don’t know if the price went back up yet.

Since Aperture does not support layers I’m afraid I don’t see how you could do this.

There are third party apps like Perfect Layers, it’s about $100 I think.
Pixelmator does not work well with Aperture as it does not like the PXM format. Bummer.
Photomatrix is $80 and offers HDR loveliness.

Apple has a page devoted to these options.

http://www.apple.com/downloads/macosx/aperture/

Unfortunately they’re all at least 2 years old and the prices are not given, you have to go hunting for them on the dev’s pages.
(Therein lies the core weakness of Aperture when stacked up against Lightroom, which has a far stronger Pro user base and a new version…)

As the others have noted you’ll need to grab a plugin of some sort to handle layers. Pixelmator is not a bad pickup, but it doesn’t play that well with Aperture. Another option is a fairly new option from Topaz Labs called photoFXlab (http://www.topazlabs.com/photofxlab/). It’s both a plugin and standalone app and offers simple layers work. You don’t need the other add-ons (though they add greatly to the entire package) and I’ve found it plays nicer with Aperture than Pixelmator.

When I recommended Pixelmator, I wasn’t aware of the “doesn’t play well with Aperture” issues. The issues are explained here: http://www.gabrielponzanelli.com/blog/apple-aperture-and-pixelmator-inte…

It can still work and do the job but it takes more work at the moment to get the edited file back into the Aperture library which is mainly an inconvenience.

Wasn’t aware Perfect Layers was free now. Thanks for that.