Ciao Simone,
DDE is ages old, it's past!
What about reusing a free doc/docx to RTF converter? See HERE as an example.
If you really want/need to program your way:
1) Load Word signatures into your Uniface repo with /sti; see documentation for examples.
2) Build a very small program which instatiate Word application, open your doc/docx document original document and save it as RTF. In this section you could obviously do much more: use Word VBA macro recorder for examples.
3) Later on you could fileload that RTF file into a Uniface RichTextField but keep in mind the Uniface RichTextField is customized to Uniface environment needs (like: images embedded into RTF document are not shown).
An example very similar to what you are asking for is in your PWD_EXCEL form.
Gianni