Modifying Users Administration Grid and Profiles

In our old version (3.7 SP4) we used a modified User Administration List (a page stored at /Sitefinity/Admin/CmsAdmin/Users.aspx). Our modified list included extra columns and links to activate or deactivate a user.
Our back-end user profile pages were modified as well, and included additional properties and controls (dropDownLists, checkBoxes, etc.).

Is there a way to modify the standard users grid and user profile in the back-end of Sitefinity 5.0 SP1?
In the front-end there's an easy-enough way to edit user registration and user profile templates. But we couldn't find a way to edit the back-end templates.
Any pointers will be appreciated.