Hi
Disclaimer: Im not a Drools user! - However, I would probably look at wrapping the Drools engine in a class which I could delegate to. Thus you could use the equivalent of a service task. Hence the engine hands control to your wrapper class, you package up variables, call drools as a library, re-package the result.
Note you can still use the BPMN rule task here as you can select a java class as the implementation mechanism.
regards
Rob