Are you looking to have the contents of a “subroutine” Tinderbox document included in another document? Similar to Python’s
import file1? That, in itself, is not support as far as I know. However you can “watch” a folder in the file system using File > Watch > Folder from Finder… – see more information here. This will not do much for watching other Tinderbox files (whose XML content cannot easily be used directly inside another Tinderbox documents), but you can watch other files with plain text content that describes templates, action code, etc., that can then be copied and paste into other locations in your master Tinderbox document.
In you merely want to open other files, then in addition to @mwra’s suggestion above, you can use the system $File attribute or create your own file-type arttributes:
Personally, I avoid
tinderbox:// URLs since as soon as a document leaves the File > Open Recent menu, the URL fails.