Hey Jeremiah,
yes, the Buildbotics.com controller allows exactly that. But there is no such connector on Onefinity’s software and hardware fork of the Buildbotics controller.
Buildbotics.com introduced that auxiliary connector after one user really succeeded with accessing to the solder points for STEP±{X,Y,Z,A}, DIR±{X,Y,Z,A}, Motor Enable, and Alarm on the AVR Mainboard (gerber source) and demonstrated the use of external stepper drivers (thereby bypassing the internal Texas Instruments DRV8711 stepper drivers). But Onefinity generally does not backport any new things, neither hardware nor firmware.
I had a look at the pcb, and although I am an experienced solderer, I would say, if you want to attempt this on Onefinity’s pcb, good luck! 
Better buy a Buildbotics controller that has the 15-pin D-Sub auxiliary connector.
Image 1: Buildbotics CNC Controller
Image 2: Onefinity CNC Controller
EDIT: Link to Onefinity pcb gerber file repaired.