You are kind of stuck as the built-in controller is where the current sensing and feedback needs to
be done (its usually the inner, fastest, control loop in any motor-control system).
But if you sense the supply current you'll be able to do a cruder job and throttle back on over-current
on a longer time-constant.
To explain motor control loops v. briefly:
Typically the inner loop runs around 10kHz or more, adjusting the PWM drive to achieve a programmed
3-phase current (usually measured at the windings). This is direct torque control, and it is then used by
an outer position or speed control loop (more like 1kHz rate) as the output from its PID loop. This makes
it easy to add torque limiting by bounding the control signal to the current loop.
There are some great talks on motor control on youtube here: Teaching Old Motors New Tricks - Part 1 - YouTube