Converting an image file to .psd is easy in Aperture - just export the image version with one of the export presets "PSD - Original size" or "PSD - Half size".
But I suspect, that will not solve your problem, since you are trying to create a watermark in Aperture and probably want to create a transparent background for the watermark, to give it an arbitrary shape. That cannot be done in Aperture. Aperture does not support compositing and layers. If you want to create an image of an arbitrary shape with a transparent background you need to use a program that supports compositing. Since you do not have Photoshop, you could try the free GIMP instead (afaik, there is no Lion version of GIMP; the most recent version is Mt. Lion only; you could try if the Snow Leopard version works on your system).
Any program that you own, that lets you write or draw on a transparent background will be fine. The Watermark file does not necessarily need to be a psd-file. A png will also work. You can also use Preview to convert between image formats or to annotate an image with text.
The other possibility is to do the watermarking in BorderFx an Aperture plugin.
Depending on what you want the watermark to look like you might not even need to produce a separate file you could do it all from BorderFx.
If watermarking is what you're after, check out the watermarking articles on ApertureExpert (do a google search for "apertureexpert watermark" and you'll find it straight away. And if you create your watermark images as a PNG file with a transparent background, that will render as you'd expect on the exported image. (Like leonieDF says, the watermark source doesn't need to be a PSD file.)