Try to create an entity and point to the keys in type section and create the custom fields try to point to the type section fields if the fields are not available in existing entity.
Note: It is just to create a new entity at the same level.
You mean to say i need to create a new brand Entity in the Type section. if yes, then it will fatal the database. Please correct me if i am wrong.
Create a custom entity and point that to one of the entity in type section it is like 1-m relationship. Please have a look at the Domain model of repository(PIM data model) for more information.