Does an adapter exist that can convert a rotary tool in a oscillating multi-tool one?
There are no universal solutions.  All the powerheads are proprietary (as are the batteries/chargers)

An angle grinder is too fast, too heavy and too powerful for the precision that the oscillators provide, as a class.

Any combo you would jury rig in this fashion would not be worth the time, money and frustration.  

I have the Rigid oscillator, and I think it's a bit heavy, vs the Fein single function, but I like the right angle impact driver for close quarters use

Good luck if you invent something, let us know.