Is it correct for air to flow into home from under an addition?
Radon emitted from soil can pass through concrete quite easily. Furthermore, concrete and other earth products themselves can emit radon as well.
The fresh air vent coming from under an addition will not be any better with or without a concrete pad if the radon source is not properly mitigated.
If you have a concern for radon exposure in your home the only practical solution is to have your home professionally tested and mitigated if necessary.