This is not a formal Mod...code hacks have been provided in the Links 2.0 Customization Forum...All you have to do is add another field to your links.def file and then update your links.db file.
There is a FAQ in the Resource Center that discusses adding fields to your links.def and updating your links.db file.
BTW: The better forum to post requests like this is the Links 2.0 Customization Forum.
Regards,
Eliot Lee