You can invoke the form with parameter which shows You what You must do APPEND or EDIT.
Somethink like that:

** Key press event of the main form
DO CASE
   CASE nKeyCode == 13
        DO FORM MyEditForm WITH (nShiftAltCtrl==1) && .t. for edit, Shift+Enter->Append
...
ENDCASE

You can store that parameter in propertie of parent form to know what you must do if the user press SAVE button