It is possible by Adding OpenType Features, but not many programs support Glyph Substitutions.
In FontCreator you would create your S comma as a composite glyph (or a simple glyph if you prefer), but you need to use another program to add the glyph substitution.
You can also use Microsoft Keyboard layout creator to define shortcuts for typing accented characters. Then you don’t need OpenType features, and can type your composite character in most applications. This will work in exactly the same way as typing Ś or É using conventional fonts.
Combining Comma Above is defined in Unicode, but S comma above is not as far as I can tell, so you would map this character to the Private Use Area and define your keyboard to type that PUA character.