Go on and do it.
But please name those external encoders not encoders, but sensors. That's the used terminus technicus. As you already told, encoders are used for measuring position and as first/second derivation the speed/acceleration. You want to measure forces and accelerations those devices normally are called force/acceleration sensors.
Anyway, this is done already in many applications (at least for force sensors), but always as special solution for a specific task.