Hi Joerg,

>I tried several supporting classes one named struct from Christof Lange and Mark Wilden.

Wollenhaupt is Lange after marriage...

>and another one called rpstruct from http://www.foxitaly.com/ecerlini/struttureapi_en.html
>but I can't get it to work

As you haven't said anything about the language the DLL is written in and after a look at the VB code SWAG it is the type housing 4 strings. DOn't remember what VBA passes for such a declaration as value. From the hip: pragmatic attack vector would be to check if you can call the DLL from VB6/classic with identical declaration. In case of success, write a small VB6 COM component, which then builds the second parameter and calls the DLL. This component is called from vfp with better control on your side via parameters or gets properties filled via COM.
Other possibility (if it is a C DLL and if concatenating strings with added chr(0) does not work) is to create a function in C to check what VBA passes into it from the C side - then fiddle until you get the same when calling from vfp.

Better to contact the dev of the DLL if available - or do some background reading on VBA types housing strings passed to a DLL.

HTH

thomas