How to change a jBPM process variable value in a Script Task without using kcontext

Issue

• Given an application requirement which it must be designed to be workflow engine agnostic. Some Script Tasks is required the ability to set the value of process variables however the use of the implicit variable kcontext is jbpm-specific, so it can not be used. How to change a value for a process variable without using kcontext or another jbpm-specific API?