They make stick-on touchpads. Here’s one: Amazon.com: ETPA Mini USB Computer Touchpad, Trackpad, Mouse, Wired, 1.1 x 1.3 inches Ergo Touchpad ETP001MTPUSB : Electronics - 0.348cm x 0.2825cm (for $60 - yowch!), so it might fit in the center spot, but I’m doubtful. If you took out the trackball mechanism you could stick it on top.