Hi Cynar,
from the software point of view, the simplest way to go is to adapt output signals from internal H-bridges on CORE2, to your external H-bridge inputs. But this possibility depends on your specific driver.
Some time ago I posted the schematic of external H-bridge that we had used internally:
The resistors and diodes are translating the 12Vpp signal from CORE2 hMotor output (CORE2 is supplied with 12V).
If you can provide the information about your external module, we can help you further 
The other way is to use PWM signal from hServo outputs - there you have 6 GPIO pins with PWM support.