You don't need to replace the hub. You can either get another Uniglide cassette, grind off the tab on a Hyperglide cassette as miamijim suggests, or simply replace the uniglide body with a hyperglide one on your existing hub so you can use Hyperglide with no grinding required. If you don't mind friction shifting you can probably go to 7 speeds without a problem. That's exactly what I did recently with an old Stumpjumper.
http://www.sheldonbrown.com/k7.html