I have a 14-26 5-speed Atom freewheel that won't thread onto a hub. Both hub and wheel are English threaded (yes, I'm sure). Not sure what the problem is--it was threaded onto a previous wheel fine, and the threads of the hub I'm trying to thread it onto are fine, or so I assume because it accepts another freewheel no problem. I haven't tried too hard so far because I don't want to damage the soft aluminum threads on the hub, but I'm getting frustrated. Should I try to chase the threads on the freewheel, even though they already look fine? And does anyone even make such a tap?

An English BB lockring is the same threading and will help shape the teeth on the hub. There's a good chance the first few threads are slighly damaged and if you can get a lockring to thread on that would help shape them back up. The threads are large diameter and fine pitch so it's very easy to cross-thread them, which will cause the type of damage to give the symptoms you describe. Just get something (freewheel or lockring) to thread on straight and you'll be fine. It just might take a good deal of patience and a small amount of force to get the threading started.