It's very easy. Let's go create a keypress event that is the same as if the user has pressed Ctrl+Enter:

 event = gtk.gdk.Event(gtk.gdk.KEY_PRESS)
 event.keyval = gtk.keysyms.Return
 event.state = gtk.gdk.CONTROL_MASK
 event.time = 0 # assign current time

 widget_that_should_accept_signal.emit('key_press_event', event)

the last line "passes" the signal to the widget we want to handle this fake event.