Hi Giulio,
You can try adding a new rigidBody to the tool body in your rigid body tree which coincides with your center point. This new body should be fixed to the tool body, and should be attached at a fixed offset transform.
Refer to the documentation of rigidBodyJoint https://www.mathworks.com/help/robotics/ref/rigidbodyjoint.html to see how to do so.
You can also refer to this example openExample('robotics/AttachARigidBodyAndJointToARigidBodyTreeExample').
Best,
Karsh
