The blpackage editor does not let you create anything from scratch. you can add additional assets into your package, or you can do what you did - modify the file on the server and add the file. but you cannot add a new entry directly from the editor.
file a ticket w/ our support asking for this feature.
One of the major enhancements that came out of BofA POC was why can't we simply get an expanded view to put the commands etc when we add external commands. The view is so tiny and error prone due to blank spaces etc.