You would need either Uniglide or Hyperglide cogs. Hyperglide may need some Dremel work or filing to make them fit.

You cannot fit 7 cogs on a 6 speed, but can change the tooth count, depending on the rear derailleur you are using.

Loosescrews still sells Uniglide cogs.

Or change the freehub body, as roccobike says.