How to add Textures to other textures during runtime

Hi, I would like to know how can I add textures to another textures during runtime as it can be seen in some games where you can add stickers/decals/images to (eg) cars. Here i found a video of a game (Street Legal Racing Redline) which has that property: http://youtu.be/pQgvmOrpZkw?t=2m . It can be seen how the user during the runtime can change the color of the game object and add several other textures to it, also being able to adjust it making it bigger or spinning it and of course the fact that he can place it wherever he wants.