The 'subparsers' object has a _parser_class attribute that is normally set to the class of the parent parser.  In the attached file I create a

    class CustomParser(argparse.ArgumentParser)

that makes a parser instance which copies all of the attributes of prototype parser.

    proto1 = argparse.ArgumentParser(prog='SUBPROG1')
    proto1.add_argument('--foo')
    subparsers._parser_class = CustomParser
    sub1 = subparsers.add_parser('cmd1', proto=proto1, help='parser based on proto1')

'sub1' is a functional copy of 'proto1'.  I think this does what you want without changing the argparse code.  There probably is a way of defining CustomParser (maybe its '__new__' method) so 'sub1' is actually 'proto1'.  But the copy approach appears to work just fine.