Well.. to do so directly would require you interacting with the software at a low level. And therefore the precise actions required would vary as it would depend on the software concerned (interacting with DLL's for example - making system calls.)
I suggest that another approach, would be to let the user select the fax driver from the available printers. Fax drivers are normally classified as printer drivers and as such can be selected from those avaialable on the sytem
Once the user has specified the driver, all that is required is to switch the default printer to the required printer and send the document accordingly. There is code around that shows how this can be achieved.
Hope this helps..
Opp.