Half the fun of these PiTFT displays is making tiny portable projects. Having a giant USB keyboard attached runs counter to that. One more piece of software called retrogame lets you connect buttons directly to the Raspberry Pi’s GPIO header and simulates key presses. We use this in a lot of our gaming projects!
Retrogame is already covered extensively in its own guide:
Keep in mind you’ll want to steer clear of the GPIO pins used by the PiTFT display — they’re indicated in that guide.