Hi, I will assume here that you are using jBPM 5.2 and that you are using the jbpm installer (it would be nice in the future if you could specify this info if you wish to have a more detailed response):
in $jbpm-installer-dir/auth directory you can find users.properties and roles.properties files. Let's say you want to add a new user called superman, in this case you would add to users.properties for example:
superman=superman
and in roles.properties:
superman=admin,manager,user,<SOME_CUSTOM_ROLE>
where <SOME_CUSTOM_ROLE> is to be replaced with your custom role name
after this change you would run in $jbpm-installer-dir
ant clean.demo
ant install.demo (or ant install.demo.noeclipse)
ant start.demo (or ant start.demo.noeclipse)
Hope this helps.