Until there will be an HSB to RGB Image node, you could probably use an ImageJ macro just for the conversion back to RGB. The complex thing is that you have to create a new image in ImageJ, convert it to an HSB stack, paste your channels in there, and convert it back to RGB:
run("Stack to RGB");
run("RGB Stack");
inputID = getImageID();
getDimensions(width, height, channels, slices, frames);
newImage("Output", "RGB black", width, height, 1);
run("HSB Stack");
resultID = getImageID();
selectImage(inputID);
setSlice(1);
run("Copy");
selectImage(resultID);
setSlice(1);
run("Paste");
selectImage(inputID);
setSlice(2);
run("Copy");
selectImage(resultID);
setSlice(2);
run("Paste");
selectImage(inputID);
setSlice(3);
run("Copy");
selectImage(resultID);
setSlice(3);
run("Paste");
run("RGB Color");
run("Make Composite");
I attached a workflow that illustrates this.
Hope that helps,
Jan