The proper way to add a resource in Sencha Architect, regardless of framework, is to add it as a JS resource.
This is most easily done by hitting ctrl-t (or cmd-t on a mac) and typing "js resource" (or some shortened string thereof to get autocompletion)...
Alternatively, you can use the toolbox, click "Resources", and drag out or double click "JS Resource"
These are just different user interfaces to accomplish the same task.
Once you've added your JS resource using one of the above methods, you need to set the URL in the config pane (at bottom right unless you've configured Architect to swap the left and right panes)...
The URL is to be set relative to your project folder. If you copy the JS file into your project root under a folder called "lib" for example, then you'd set the url config to "lib/foo.js" - where foo.js is the filename, of course.
Hope that helps!