Since the external force is in Newtons, it cannot be directly added to the input of the plant which is expecting current in Amperes. For example, let us assume the following transfer function for the plant:
This can be written as the following ordinary differential equation by taking inverse Laplace transform as:
We can incorporate the External force f(t) here as:
Taking Laplace transform again we have:
From this, it can be observed that a force 
can be provided to the plant together with the output from the controller to simulate an external force acting on the magnetic bearing.
can be provided to the plant together with the output from the controller to simulate an external force acting on the magnetic bearing. The following picture shows a screenshot of a Simulink model where the disturbance force is incorporated:
You can try a similar approach with the transfer function of your plant.
Regards,
Arnav
