It’s not possible to just plugin an API into the CMS. An API is just a set of scripts you can talk to and it will respond with what you requested.
See also: http://en.wikipedia.org/wiki/Application_programming_interface for what an API is.
GodZZila is right, use the existing URL he provides, or build (let someone build) a webpage for you on top of the KNVB API and include that in the CMS.
If you need help, just send me a message. (I can respond in Dutch if you like.)