well, i don't think you can do that..
but at the bottom of the listing, in checkout options, i think, you can add that info there, or go via 'revise item' link on ebay and add it after it's posted.. (i mean the return policy info)
i personally put my returns policy under 'terms of sale'..
if you wanted, i'm sure after you create your listing, you could go into the html, and add a returns policy section wherever you wanted. like customizing it, ya know?