I believe there are two ways to configure custom attributes:
1) first modify */opt/gluu/schema/openldap, restart the service, then submit web form oxTrust -> Configuration -> Attributes -> Register Attribute.
2) submit an LDIF of the attribute using oxTrust -> Configure -> Import/Export Attribute LDIF. I haven't tried this yet and am not sure exactly what it updates.
Is there a way to add new attributes without requiring the use of the oxTrust web app? For instance, are there flat files that can be edited to complete the entire schema update & attribute registration prior to starting the service? We're inquiring as we prefer an approach to configuring custom attributes that can be scripted for use in configuration frameworks (like Ansible, Chef, Puppet, etc.).