Is it possible to shoot M-RAW or S-RAW files with all their advantages and convert them into RAW files later? Is there software available for this task?
Once you discard information before saving a file you can not later retrieve it.
M-Raw and S-Raw files are lower resolution versions of the raw file created by the information produced by the digital sensor on the camera. The information from multiple pixel wells is averaged into a single value for each "pixel" of the lower resolution files. Once that information has been combined, there is no way to determine which parts of it came from which of the source pixels. It would be like trying to see the color of several different paints by looking at a can of paint in which they have all been mixed together into a single uniform color.