Luis, you have the terms reversed. The old clusters were called freewheels, and the new ones are called cassettes. Anyway, you can't put the 6-speed freewheel on the 10-speed freehub, but you might be able to use the freehub as it is. The way to find out is to try it. Do you have tires and tubes?

Given that you have these wheels on hand already, they're the best thing to try next. They cost no extra money. They might work!