I know this won't answer your question, but that is INSANE . . . there is no way I'd let anyone push my car around without steering or brakes.
On non-start button cars, the only way I know to do that with an automatic is to have the ignition unlocked, which requires that the key is left in it. Not sure how you'd do that with the start button.
2009 E90 328i - Space Gray / Black-Ette / Aluminum / 6MT / Xenons / BT / iPod / Legal Tints / Blue and White CF Roundels / Clear Bra / Done Modding (daily driver)