To answer your questions
1) There a two products that I am aware of that assist with rules. One is open source and part of the Drools projects : Excel Assistant Link http://drools.org/Decision+Tables, the other commercial - DRL Assistant http://www.zymose.com/faq.html. If any other readers are aware of similar products , please let me know.
2) Yes Drools is open source , and not only can you change it but it is then your choice over whether you release these changes to the community or not (more info at www.drools.org/Licensing)