Replace hub from old cassette to new fixed
I've seen this done and mostly work out ok, but I still wouldn't recommend it for the commented reasons.  If do want to attempt to reuse the spokes, you'll need to buy a hub with as close to the same flange size as possible.  If you go with a wider flange, you'll end up with spokes poking through your rim that you'll need to cut and possibly rethread.  If you go with a smaller flanged hub, the spokes may not be long enough.

Spokes are the answer to a question involving the effective rim diameter, flange width, pattern and a few other variables.  If you change any of those variables, the answer will be different and you'll need different spokes.