You can add new fields by going to each window, and under the window's menu, go to Tools > Tab Management (e.g. Accounts > Tools > Tab Management).
Tabs and Fields Management menu
Create a new tab in the Tab Management window. You can also rename user-defined tab by selecting the relevant tab and clicking the Tab Properties icon.
You can manage which tabs are displayed for the entity using the Tools > Tab Management window. Please note that the 'General' tab cannot be removed from the display.
Tab Management Window
Then you can go to Tools > Field Management to create new fields for your new tab. You can also use existing system fields which will be displayed in the user-defined tab.
Field Management Window
You can create as many new tabs as needed and create as many new fields as needed. You can also design the layout of the tab page as best suits your needs.
In any case, it is always recommended to backup your Commit installation before performing any customization!