Any changes to the blocktype after it has been created must be done directly in the code of the blocktype (unfortunately you can't change a blocktype's field definition via the dashboard interface after it has been initially created).
I'm not sure if you're asking to add another editable field or just hardcode a hyperlink... if the former, you will need to create a new blocktype in the dashboard and use that instead. If the latter, you can just add your link (or whatever you want really) to the blocks' view template, which can be found in SITEROOT/blocks/your_block_handle/view.php

-Jordan