Here's a way of passing an optional-like argument to a subparser:

    parser = argparse.ArgumentParser()
    subparsers = parser.add_subparsers(dest='cmd')
    sub1 = subparsers.add_parser('cmd')
    sub1.add_argument('foo',nargs='*')
    args = parser.parse_args('cmd -- --def 1 2 3'.split())

producing

    Namespace(cmd='cmd', foo=['--def', '1', '2', '3'])

The  '--' forces the parser to treat '--def' as a positional.  If nargs='REMAINDER', foo=['--', '--def', ...].

But the following subparser definition would be even better:

   sub1.add_argument('--def', action='store_true')
   sub1.add_argument('rest',nargs='...')

Here the '--def' is handle explicitly, as opposed to being passed on.

You don't need the whole subparsers mechanism if you are just going to pass those arguments (unparsed) to another program.