Merging HSV planes to get modified S plane

edit

My question still remains, but what I guess you are looking for is

Mat image_BGR, image_HSV;
imread("/path/to/image", IMREAD_COLOR);
cvtColor(image_BGR, image_HSV, COLOR_BGR2HSV);
vector<Mat> channels;
split(image_HSV, channels);
Mat S_channel = channels[1].clone();
// do some processing on the S channel here and store it back in S_channel
// Make a 3 channel image again exactly like the modified S-channel looking 
Mat merged(image_HSV.rows, image_HSV.cols, CV_8UC3);
Mat in[] = { S_channel, S_channel, S_channel };
int from_to[] = { 0,0, 1,1, 2,2 };
mixChannels( in, 3, &merged, 1, from_to, 3 );

Good luck!