How can I connect to an unthreaded galvanized pipe stub for a garden hose?
They make fittings that we call "Moody Couplings"; this may be a regional or colloquial term but probably evolved from a manufacturer's name.



These are designed for pressure pipe, the size designed for 1/2" galvanized pipe also fits 3/4" copper. These are not designed to be installed in locations where a leak would not be evident/fixable, and they do require a relatively smooth outside surface (but are a bit forgiving in that respect). If you do this, I recommend not covering it up, so you can monitor.