I am adding a on to a bathroom in my basement. I want to put a sink a tub in. The main issue is that the sewage line under the concrete floor is cast iron. Can I connect pvc to castiron with a boot and then cover back up with dirt? Are there an issues with do it that way? Is there a better way?

You can use a no hub coulpling designed just for that purpose. It should say right on the edge of it, 4" pl x4" ci. (If 4 " is what you need) pl stands for plastic and ci stands for cast iron.