I've created a new page type called IncludeHTML. In this pagetype I've added a File field, and a CMSfield of the type FileFrameField.
This allows me to choose a html-file uploaded to the assests folder or upload one from my computer.
My goal now is to get the contents of this html-file included in my IncludeHTML-template (IncludeHTML.ss) wherever I choose to put it. As an example $IncludeHTML ... should output the source code from the html-file chosen in the backend. This would ease my work load when including image maps etc.