In the attached patch I modified 'add_parser' to take a 'parser' keyword parameter.  If given, it is used as the parser, rather than create a new one.  Thus an existing parser, or one created with a custom ArgumentParser class, could be used as a subparser.  

In this sample script, a parent parser is used as subparser in 2 different ways - via the parent mechanism, and with this new add_parser parameter.

    parent = argparse.ArgumentParser()
    parent.add_argument('-f')
    parent.add_argument('bar')

    parser = argparse.ArgumentParser()
    sp = parser.add_subparsers(dest='cmd')
    p1 = sp.add_parser('cmd1', add_help=False, parents=[parent])
    p2 = sp.add_parser('cmd2', parser=parent)
    parent.add_argument('test')
    assert p2 is parent
    assert p1 is not parent
    print(parser.parse_args())

This change passes existing unittests.  I don't think there are any backward compatibility issues.