Yes one can add cities as in zyGrib.
The location of the 'data' folder and the 'gis' sub-folder with the city files can be found in the user preferences area according to your operating system.
In windows it is in <USER>\AppData\Roaming\openGribs\XyGrib\data\gis\
In Ubuntu it will be in /home/<USER>/.local/share/openGribs/XyGrib/data/gis
Enjoy,
David