Hi Jaber,
At this time, I really don't know how to register custom functionMapper for juel. In jBPM 4.4 trunk, there is a org.jbpm.pvm.internal.el package for the expression operations. Maybe you could achieve a custom FunctionMapper and register it to the JbpmElFactory.
I think opening a new issue for this feature would be a good idea. Could you do it?