actually everything is possible. All you have to do is try simple ways to create those cities. Like placing some box or placing planes with alpha, or place city picture in your sky box etc. I always use some box and some building textures on it. Then i just place them everywhere i want 
Literally just box. Even BSP’s will do the work. Create 4 or 5 bsp with some material with building faces on it. Then aplly them on your bsp convert to geometry and it will do the work If you want more cheap (less performance) way you can use plane and of course material with building faces. And just apply them on your plane then place it everywhere. If you want to place them quickly create procedural blueprint.