1) did you check the drools docs? ->
http://www.jboss.org/drools/documentation
There also 3 books about it that you can read to get started
You can integrate jBPM with another rule engine, but it doesn't make too much sense due the fact that jBPM is already integrated with drools. So if you integrate with another rule engineyou will end up having 2 rule engines :)
2) All the components are provided by the jbpm project and the drools project
Look at Drools Guvnor for the Process, Form and Rules repository.
And yes.. you will need a database that is used by the engines to store the runtime status.
Check my blog salaboy.com to find some slides about the architecture.
3) Why do you want to store rules in XML? that was deprecated long time ago. If you want to go in that direction, you can always create an exported from DRL to XML and viceversa.
Cheers