Can I link to documents outside of SourceForge?

SourceForge has the concept of a "url doc" to support this very usage. When creating a document in SourceForge, simply choose this type from the Create screen and then enter the URL that references the document in the existing external system. This will create a "placeholder" document in SourceForge that can be associated to, reviewed, etc as if it were a normal document, while maintaining the document's actual contents in the external application.