For night photography the 20D doesn't have live view. AF doesn't work at night so you'll have to focus manually and live view is really helpful. One trick could be to focus on city lights far away and never touch the focus afterwards (unless you're taking pictures of something that is close) using back button focus.
Night photography also benefits from good high ISO performance. If you want pictures of the night sky don't want star trails you'll need a reasonably wide and fast lens and good ISO performance. You can still get usable shots through stacking, but the worse the low light performance the more work you have to do.
For taking shots of the city at night
here's DPReview's long exposure part of the 20D review. Hot pixels can be an issue in those older DSLRs.
If you can find some cheap glass though it's probably a nice and cheap way of getting into photography. If you like it you can buy a higher end Canon body with the features you want or switch system and sell your old gear (lenses hold their value much better than camera bodies). I'm not sure what the price is, but if you can find an old 18-55 for a low price then that is probably a nice starter lens. They're not great for low light, but very cheap and get the job done.
You can't make a speedbooster for an SLR mount. No space for the optics.