To quote dev diary 11:
"The greatest thing to come out of the first Cities in Motion is the wonderful community around it. Mods, user created maps and other customizations made by the community have brought so much more to the game. This means a lot to us and we want to support our fans’ efforts. We want the community to be happy with the new game and be able to once again make it their own. The challenge has been that we use the Unity Engine in the sequel, which has a much better performance than the engine used in the first game, but has some limitations concerning mods. We have worked our way around the obstacles by providing a detailed map editor and adding a scenario editor and modifiable rules lists. Those last two features will be included in a free update after the game is published, just to give us enough time to polish them to perfection while you enjoy the campaign, multiplayer and map editor."
So from that I gather that they fully support modding in theory, but the unity engine makes modding difficult. Hopefully people will get around this and still make mods, I was planning on converting some CIM 1 vehicles (both player and traffic) and buildings to CIM 2 to give some more variety.