This paper presents the kinematic design and development of a two degree-of-freedom (2 DOF) spherical 5-bar thumb exoskeleton to augment the FINGER rehabilitation robot, which assists the index and middle fingers individually in naturalistic grasping. The thumb module expands the capabilities of FINGER, allowing for broader proprioceptive training and assessment of hand function. Design process started by digitizing thumb-grasping motions to the index and the middle fingers separately, recorded from multiple healthy subjects utilizing a motion capture system. Fitting spheres to trajectory data of each subject allowed normalization of all subjects' data to a common center and radius. A 2-revolute joint serial-chain mechanism was synthesized (intermediate optimization step) to reach the normalized trajectories. Third, the two resulting grasping trajectories were spatially sampled as targets for the 2-DOF spherical 5-bar synthesis. Optimization of the spherical 5-bar included symmetry constraints and cost-function penalties for poor manipulability. The resulting exoskeleton assists both flexion/extension and abduction/adduction of the thumb over a wide range of motions. Consistent with FINGER, the parallel structure of the spherical 5-bar places the actuators at the base of the module, allowing for desirable characteristics including high backdriveability, high controllable bandwidth, and low mechanical impedance. The design includes an adjustable thumb cuff, accommodating different hand sizes. Fit and function of the device were tested on multiple subjects, including survivors of stroke. A proportional-derivative force controller with gravity and friction compensation was implemented to reduce resistance to motion.