If it wasn't wrong or stupid, what would be the point of doing it?
It depends on the width of the dropouts. Sheldon Brown goes into more detail here:
http://www.sheldonbrown.com/frame-spacing.html
Basically, you have three choices:
Buy a rear wheel with an ISO-threaded hub and stick with a 5-speed freewheel. IRD makes modern (but pricey) freewheels that will shift like a modern cassette, or you can get one with older-style teeth from Sunrace or Falcon. If you want more than 28t for your low gear, you'll need the IRD or an old Suntour cassette.
Use the same ISO hub with a longer axle and some spacers to mount a 6-speed freewheel. This will let you use a cheaper Shimano freewheel.
Cold-set the frame and use a cassette hub. You'll probably have trouble spreading enough to get much more than a 7-speed cassette on there. The axle will be stronger since the outboard bearings are at the edge of the hub instead of the wheel.