Hi Juan,
you don't need to use the JBPM4_ID_USER, JBPM4_ID_MEMBERSHIP and JBPM4_ID_GROUP tables from jbpm4, the only way jBPM4 interacts with identities is by using String IDs, like for example the taskService.findGroupTasks(userId) or task.getAssignee() methods.
So you can implement your own Abstraction layer for whatever identity backend you have.
Having said that, of course you are free to use the included tables and the jBPM4 identity component however there might be some features missing.