Drools - Ability to define rules at runtime?

Read the documentation and check out the tests about "KnowledeAgent" to see what you can do and how you can do it.

Yes, Drools is completely dynamic. You can add/remove/replaces rules at runtime. All the existing sessions are automatically updated in order to keep consistency. Check the documentation/javadoc:

http://www.jboss.org/drools/documentation

And here you can see some integration tests:

DynamicRulesTest.java