Hi Filippo,
I'm not sure why you need to fake-up/model a force as a body-weight. Fundamentally, bodies have mass properties and articulation (several joint types) that connects it to the rest of the multibody system (Composed of Bodies, Joints and Constraints) while an external force is an abstraction for how the environment interacts with the model, as such there's no way to specify the bucket as an "external force" since it's not "external" anymore when it's a body in the model. You can change the mass and run ID with your desired trajectory but I'm not sure about the rationale, considering that you can specify and apply "external forces" as described in the example I sent earlier.
Hope this helps and best regards,
-Ayman