docs are really really great, it’s hard not to find a solution in there...
Adding the tab is easy with Form Customization.
http://rtfm.modx.com/display/revolution20/New+Tab
You’d have to use a plugin to add your own content to it. I’m not sure how you’d put content on a particular tab, but I’m pretty sure it can be done.
http://rtfm.modx.com//display/revolution20/Adding+Custom+Fields+to+Manager+Forms