as was before mentioned, mild steel *can* be used in a blade billet, but it will suck up carbon and you'll have to avoid having it on the cutting edge.
I've used spring steel in laminated steel before, and I didn't run into any sort of problems welding or working with it. What you will have to look out for is combining steels that cool in different ways or behave differently at high temperatures. Putting an air cooling steel that crumbles at high temp in your billet can be very tricky.
Just take a bunch of steel you have laying around, tack it together, weld it up, and see what happens. Don't be afraid to use scrap steel for practice!