Thanks for your enquiry.
Unfortunately, as far we know, it is not possible for one C3PO to call another C3PO’s custom action menu commands. Formativ is a GroupWise custom third-party object (C3PO), ie. it integrates with GroupWise.
If your GroupWise integration (or other software) exposes a COM API, you may wish to experiment with a Formativ applet that drives the COM API.