Can I use a DAC with an L293D driver?
No, you cannot. A DAC cannot provide enough current to either directly drive a motor or to power a motor driver (which is basically driving the motor directly through a switch anyway).

A motor driver requires a PWM signal to rapidly switch a high current source on and off to set the motor speed.

You could feed the DAC output into the input of a voltage-to-PWM converter chip which would then feed the motor driver's EN pin, but that is pretty pointless when you could just use a small microcontroller to receive the digital speed value and directly generate a PWM signal.

After all, that is the kind of job that small microcontrollers are designed for...