Hi electronicscafe,
While it is not possible to add additional links to the "UserLinks" menu on the description editor toolbar, you can actually create whatever links you'd like when creating your auction descriptions.
To create a custom link, please type the text that you would like to become a link, highlight the text with your cursor, and then click the "Create/Edit Link" button on the editor toolbar...this button has an icon of a globe and chain link on it. Then, just enter the URL that you would like the link to point to in the "URL" field and click "OK".
-Mike