I use an L293D (H-bridge board) for exactly this to power my doser pump motors. The pins are entered into reef-pi as 0,1,2,3,4, etc just like the rpi driver, but there's more of them. You'll need 6 pins, ENA, 1, & 2, and ENB, 3, & 4 - which for me are 0,1,2,3,4,& 5 on the PCA9685. If you're powering something like a motor, EN will be the "power" switch and then you'll either end up with 1 dummy pin that's always set to "off" or be able to switch between forward and reverse by turning one high and one low and vice versa.