As of March 2015, the Ninja kitchen product line does not include a device that is compatible with K-Cups. Ninja exclusively makes blenders, food processors, fryers and slow cookers at this time.
K-Cups are used with the proprietary Keurig coffee brewing device. Ninja does not currently have a similar product. Ninja's single-serve device is a pulse blender. The K-Cup coffee would need to be brewed before adding it to a Ninja blender as the grounds from the cup would be unpalatable.
Cuisinart, a company also better known for blenders, makes a single-serve brewing system that is able to use K-Cups.