If you were to take one of the wheels off a grinder can you put a different accesory on it? ie a flap wheel or star sander? Never having had one I don't know if it's possible.
This is entirely possible. You must have an accessory with the correct arbor size or a bushing to bring it down to the arbor size. Make sure whatever accessory you use is rated for the speed of your grinder or you may have something flying apart. WARNING: Could be very dangerous otherwise. (That is where a variable speed grinder could come in handy)
If I may suggest buy one with variable speed. Could be handy in the future if you are sharpening tools. Need slow speed. Lowes has a sale on them. Delta brand very good!
(good question if you ask me) I bought a grinder with that very thought in mind. It was a Ryobi. Turns out that it really isn't practical to change out the wheels for other things. The Ryobi, at least, isn't really built for that.
I ended up buying a drill stand/holder at Harbor Freight for about $20. It hold my handheld drill horizontally (or vertically). I then put the flap sander etc. in the drill. This works real well for me and the price was right.
They sell stuff for just what you are wanting but they seem to be overly expensive at around $150.