I would suggest that you get the custody dealt with first. If you agree to everything you can just have it drawn up and enter it in the court. Then you can do a modification of support. Have you browsed through the information in LawInfo's Free Legal Resource Center to learn more about your issue yet? See:
http://www.lawinfo.com/consumer.html. You can certainly try to speak to a lawyer to determine what legal options may be available. In the meantime, you may be able to learn more on your own. Search the "Free Legal Resources" tab, or browse the Consumer Resources. Good luck.