Hi Roman,

Did you check the help?

http://producthelp.sdl.com/SDL_TRADOS_STUDIO_2015/CLIENT_EN/screen_layout/Customizing_the_Ribbon.htm

This part is probably relevant:

Limitations

In the Customize Ribbon dialog box, you can add new tabs and groups and move them around to change their display order on the ribbon. Currently, this imposes the following limitations:

• you cannot move the default Home, View, Add-ins or Help tabs. Similarly, you cannot move the groups or actions that these default tabs contain.
• you cannot move custom groups within the default tabs. This means that inside default tabs, you can only add custom groups at the top of the existing ones. On the ribbon, these will be then be displayed on the left hand-side of existing groups. However, within your own tabs, you can freely move your custom groups.
• you cannot add a number of dynamic, context-dependent commands. For example, some Formatting, QuickInsert and Review actions like adding comments may depend on the type of file to which you will apply these actions and cannot be made available for ribbon or quick access toolbar customization.