With some limitations, you can already do this by tweaking stuff behind-the-scenes.
Go read jak's tutorial about making bigger boobs:
http://www.sexsim.com/forum/index.php/topic,40.0.htmlIn the xml files, look for <Hair>xxx</Hair> and <HairTex>yyy</HairTex> You can mix-and-match the xxx and yyy with values from any of the ladies you have installed (though the textures/colours seem to be hairstyle-specific). So, you can put robyn's hair on layla, or vice versa. Presumably, if you have other ladies installed, you can mix up their hair, too.
Hopefully someday we'll be able to come up with our own textures/colours.