Seems like I have to create a Technical Rule (DRL) inside the drools-guvnor,
by doing it this way, i can specify my import line.
And I manage to get my rule works without calling fireAllRules, maybe this is caused by my architecture where I use JBPM as service by managing the process via jbpm-console REST service instead of embedding the JBPM engine in my own application.
my test rule that finally works looks like this:
processInstance : WorkflowProcessInstance()
eval(1 < 18);