I did, I don't see any straight answer to my question.
The tag with variables maps variables from process to task instances, but it doesn't create new variables. The variables have to be already defined in the process. I need a way to "create" process variables without calling:
void ContextInstance.setVariable(String variableName, Object value);
void ContextInstance.setVariable(String variableName, Object value, Token token);
variable initialisation is not yet available in plain jpdl. you'll have to write your own action handler to do this