Is it possible to create a link from an external application to a specific document inside a DT database?
No, except through the mediation of AppleScript or perhaps an Automator action. And you can perform a Lookup in DT/DT Pro of selected text from other Service-aware applications.
Of course, DT Pro has several means of sending information to other applications, through Export or via scripts. And DT/DT Pro can link to a specific document that’s open (in many other applications) via scripting.