You can probably weld them. My guess is it is some type of chromoly alloy, perhaps varying a bit by brand. I'd do some rudimentary heat treating and tempering of the teeth afterwards.

As njkayaker mentioned, there are a couple of brands that make 7, 8, 9, and 10s freewheels. DNP Epoch is one that provides 11T. But, unfortunately, they don't do tight gearing throughout the sprockets. It is also a relatively large freehub body, and thus not practical for building tight gearing cassettes.