If you are talking about a whole field and not just an external link inside of a rich text field, you should create a custom field based on the documentation to do so in SDN:

http://sdn.sitecore.net/Articles/API/Creating%20a%20Composite%20Custom%20Field.aspx

You can inherit most functionality from the current link field.

Extending Internal Link and External Link to allow selection of Images
How do I use the command key in MacVim? For example I would like to be able to press Cmdt to open CtrlP or Cmdn to open NerdTree.
