Is it possible to install SuperTuxKart and 0 A.D. in Retropie ?
While it isn't directly possible to install 0 A.D in retropie as a system, once you are in emulation station, if you press F4 on your keyboard it will bring you to the terminal. You should then be able to run
sudo apt-get install 0ad and install the game. You can then launch it by running the command
Unfortunately I do not think it's possible to install SuperTuxKart though because the Raspberry Pi does not have the proper graphics support for it.
raspberry pi 4 does now support opengl es 3.1 https://www.cnx-software.com/2020/02/03/raspberry-pi-4-opengl-es-3-1-conformant-vulkan-drivers/ so stk and other games could now be made to be compatable with it.