Can you make an agreement with someone not to pay child support, before a child is born/conceived, incase of a divorce?

The answer is no; first off, any informal agreements that you reach are generally not enforceable unless reduced to a formal stipulation and order and approved in court; further, an agreement not to pay child support may arguably be against public policy and against the child's best interest; child support us a right that belongs to the child, not the parents, which is why it cannot be denied, waived or limits out on the amount or duration.