Game launched on
Steam today, not sat down and gave it a solid try yet but a quick look in the menus revealed a Level Editor so you can build custom maps!
Game seems to support Steam Workshop, and looks like custom maps are stored in "Steam\steamapps\common\CTU Counter Terrorism Unit\CTU_64b_Data\Maps_Custom".
If the community wants another place to host maps either for backup reasons or just not fond of Steam Workshop for whatever reason we are happy to do so.