Servos are literally just a motor, a gearbox, and an electronic speed control stuffed into one small package. They can be used as directly as drive motors so long as A) they are modified for continuous rotation, and B) they don't draw too much current.
Most (but not all) servos can be modified for continuous rotation, an internet search will return many how-to guides. Due to their popularity in robotics, you can also now buy pre-made continuous rotation servos that require no modification. Pololu has a good selection but you can find them for sale at many other places.
The TCB's onboard 5 volt regulator can supply up to 3 amps total - this includes to the servo outputs. For small model tanks (such as 1/24th scale) this would be adequate to power servos as main drive motors. Select RC Output for the Drive Motor type on the Motors tab of OP Config and attach two continuous rotation servos to servo outputs 1 and 2 (left and right tread).
If you need more current it is possible to power servos from a separate supply. The TCB only needs to share a ground connection with the servo as well as provide it the signal. You can remove the center pin (positive) from the servo plug before connecting it to the TCB, and then connect a different voltage source to it (the second voltage source must share a ground with the TCB). Good options for the second voltage source would be an aftermarket BEC powered directly from the main battery.