Modular boost system for a jack

A modular boost system can be added to a jack to enable greater lift capacity of the jack. One example can involve a jack that is positionable at a well surface for moving a tubing string through a wellbore in a subterranean formation. The jack can include a frame and an adjustable plate that is alternately (i) attachable at a stationary location in the frame for fixing the adjustable plate at the stationary location or (ii) detachable from the stationary location in the frame to enable the adjustable plate to vertically move within the frame. A boost cylinder can be removably coupled to the adjustable plate and a base for selectively supplying an additional lift capability to the jack.