Just to make it clear, do you want to write text to the message after the GroupWise signature is inserted once you press the send button?
If that true then you can not edit the message as the Formativ send event will trigger before GroupWise insert the default Signature. GroupWise insert the signature at the last stage of the send process.
Another thought, you can fuzz the GroupWise client by adding the “>>> ” (reply/forward marker) before your text so GroupWise will add the default signature before the reply forward marker.