As long as you don't change/delete any of the existent objects in that schema (like tables, views, packages, triggers, indexes, sequences, and so on) that are managed by the Agile Platform, there shouldn't be any problems in adding manually custom made objects.
The Deployment engine of the Agile Platform only compares the managed data model of the applications, and acts upon it's differences, meaning that it won't change or delete existent objects from that schema that aren't managed by the Agile Platform.
However, we would recommend to implement your stored procedures, packages and views on an different schema, for data isolation purposes, and for minimizing the change for commiting errors while changing the objects in the HUBADMIN schema. This way you would be able to simplify the data import/export operations (if any), in the situation where you want to segregate the managed data model, from the custom datamodel, and you will avoid operatiosn prone to error on the HUBADMIN schema.
Consider that on your decision to reuse the same schema.