If you want to add an option to sort by a custom field as well as the regular ones, just add the option to the selector in the template before the </select>:
<option value="fieldname">field description</option>. If it's a listing field, this will be in Admin Panel -> Themes -> Manage Templates -> Sort Listings.

If your field is named blah, this would become <option value="blah">blah</option>