You should be able to override the IdentityService with your own Implementation like this:
- extend org.jbpm.pvm.internal.svc.IdentityServiceImpl and add whatever methods you need
- instead of <identity-service /> in jbpm.cfg.xml use this:
<object class="your.custom.identityServiceImpl">
<field name="commandService"><ref object="txRequiredCommandService" /></field>
</object>