Can radiator caps with a valve be modified to make them into a valveless cap?

Yes I believe so, remove the valve and spring and make sure it's got a top seal, or put one in and it's done

The old Austin/Morris 1100/1300s used a blanking cap on the radiator and a pressure cap on the expansion tank so if you want a blanking cap one of those would do if you can find one.

I think I can buy a blanking cap easy enough,but I've got three of the type with a valve so was thinking of modifying them.

yes.