i have fiddled around searched the forum and had to subsequently find out that there is no actual command for creating Ngons for SubD, the command bar yields a command for mesh which does not want to work on SubD (why?). It seems MergeAllCoplanarFaces does this on the entire object and after scratching my head for a further while i figured that MergeFaces does result in creating one single Ngon.
We really need more consistency in the wording guys