>With my old Fox2x app, is it possible to call a DLL that I have built with VFP9 ?

Not directly, but there's a possibility to have a queue app in VFP9 which would call that dll (or do the job by itself), and would communicate with the Fox2x app via a queue table (a fox2x table, of course) -- using a timer to check for new records to process. The Fox2X app would write requests into that table, and then wait for the request to be processed. The results would be in the same table.

Clumsy, I know, and besides if you can change the fox2x app then why don't you just replace it, but still - it's a possibility.