Is it possible to add external website links into the perch navigation?
Yes - add it as a new page and select "page already exists".
I can't see the option in Runway
You didn't post your diagnostics, so I presumed Perch.
In Runway, create the page, and then on the edit form select the master page as "Local file" and enter the URL as the path.
Apologies, I should have said, can you out a target _blank on the links?
You could manage that via your template, perhaps with a page attribute to set which items should open new windows.