You can! Here's how you do it:
1. In your rulebase create an entity with the same name as the one defined in the Module.
2. Make sure that entity has the same text and public name s the entity in the module
3. Make sure that the containment relationship for that entity has the same public name (also reverse public name) as the entity in the module.
4. Add a new attribute.
The trickiest part of this is getting all the entity and relationship public names, but OPA will give you helpful errors messages to resolve this.