Convolution involving turning each pixel value to the most represented pixel value of the neighbourhood
This is a non-linear filter, the operation being performed is not a convolution and cannot be represented by a filter kernel. It is sometimes called a mode filter, by analogy to the median filter, since the value taken by a pixel is the mode of the distribution of the neighboring values. OpenCV does median filtering (cvSmooth with CV_MEDIAN parameter), I don't think it does mode filtering though.