Ed, thank you for your attention to the problem

I also found a solution implying a little bit of VB programming:
***
Private Sub Document_Open()
ThisDocument.CustomDocumentProperties("Path") = ThisDocument.Path
End Sub
***

On opening the main document this macro creates a property "Path" (and fills
it with the path to your main doc), which may be used in the includetext
field, e.g.

{ INCLUDETEXT "{DOCPROPERTY Path}\\FileToInclude.doc" }

where "FileToInclude.doc" is the Filename (without path) of your file with
the text to include.

It works (at least in Word 2003), although the path contains single
backslashes instead of doubles.