"fixed" freewheel, variable gear?

But while thinking about it during a ride one day, I got to wondering if
anyone has tried fixing the freewheel (or freehub) but still having a
cassette and multiple gears?

So why is this a silly idea? Has anyone tried it? Suggestions for how to fix
the hub?

thanks,
byron

Apart from snapping off the derailleur, simply locking down the
freewheel doesn't get over the problem of reverse chain tension
unscrewing the freewheel from the hub. If you want a fixed wheel
without a new wheel, there's an expensive converter which puts fixed
cog and lockring threads on a shimano or compatible freehub, and a
similar product is also available for threaded freewheel hubs. If you
really want a multispeed fixed, you need to invest in some rare and
very expensive retro parts from Campag or Sturmey-Archer.

But why make life complex? Just rebuild your back wheel around a track
hub and learn to pedal fast!

Good and appreciated advice from everyone; thanks. I'll just build a single
fixie and fly.