Is it possible to manipulate the content of a CMS class e.g. SecurityAdmin without actually touching the file?

For instance... can I have a file in my "mysite" folder called e.g. MyMemberExpiries which extends SecurityAdmin and adds a tab to the actual SecurityAdmin file next to Members and Permissions.

Then fill this tab without physically altering the SecurityAdmin file.. sort of how you can use extra_menu_items to add menu items to the main CMS navigation?