I think this can help you:
public void setVariable(Long processInstanceId, String nome, Object value) {
TransactionManager transactionManager = jBpmConfiguration.getTransactionManager();
try {
transactionManager.begin();
RuleFlowProcessInstance processInstance = (RuleFlowProcessInstance) JBpmConfiguration.getInstance().getKnowledgeSession().getProcessInstance(processInstanceId);
processInstance.setVariable(nome, value);
transactionManager.commit();
} catch (Exception e) {
logger.error(e.getMessage());
}
}