OK,
Firstly, changing all the metadata for Record Series, Category, Folder is a complex task and represents invasive changes to the Alfresco RM module. Many of these metadata are used to drive RM-specific behaviours in both the UI and in the back-end layers. So major changes to these metadata might mean rewriting much of the module. Not impossible, of course, but not a trivial task.
Maybe you just want to add some new metadata to these types. You can do that using custom properties and/or custom associations available as a User Dashlet.
Don't forget that Alfresco's RM module is designed to be used exclusively through the Share web interface. The web (JSF) interface is not intended to be used for RM and indeed should not be used. It sounds from your posting like you are using that interface.
There is no way AFAIK to "add an aspect" through the RM Share UI - at least not as you describe it.