Thank you for your question. Off the top of my head, there are a few options to implement the ball screw:
1) There is an "ideal rotation to translation gear" component, found under 1-D Mechanical->Rotational->Common in the component library, which can be set up as a ball screw with 100% efficiency. The relevant equation for this component is r taua + fb = 0 where taua is the applied torque and fb is the applied linear force. The parameter r (referred to as the "transmission ratio") corresponds to 2*Pi/l where l is the ball screw lead. For more details on this component, please take a look at its help page.
2) Under the same section in the component library, there is a variation of the ideal rotation to translation gear component called "Wheel Axle". It operates practically the same way as the ideal rotation to translation gear, with variations in the way it is parameterized. Here, the force equations is tau_r = r fT dir, comparing this to the ideal rotation/translation component, you will see that the parameter r here is defined as the reciprocal of the other "r" (plus the ability to apply a direction factor, dir). Once again, please refer to the help page for more details.
3) If you want to model a non-ideal ball screw (with less that 100% efficiency), you could construct it using a combination of joints for coupled linear and rotational motion:
3a) with a revolute and prismatic joint combination, coupled through appropriate motion and force/torque drivers.
3b) with the cylindrical joint, again coupled through appropriate motion and force/torque drivers.
4) Finally, you can, of course, implement the ball screw as a custom component (using the desired governing equations).
I hope this helps! Please let me know of your progress with any of the above suggestions.