You don't need to trigger the key event, just respond to it as though it were any old method call from the button. After giving it a target and a selector, give it a key equivalent of @" "
and respond to the key press accordingly:
- (void)someAction:(NSButton*)sender event:(NSEvent*)event {
//...
if ((sender.state == NSOnState) && (event.keyCode == 0x32)) {
//Zoom
}
//...
}