In answer to your questions;
1 Yes you can create custom fields for those at ESM > Properties > Custom Types.
2 Currently there is not a way to do logic or math on captured fields. That would be a very useful feature so please submitt a PER at https://mcafee.acceptondemand.com/index.jsp
3 As there is no math ability that is difficult to do. Maybe you could use an expression to at least put the 2 values in to a field and then you could add them manually?