Hi,
OK then you are going to have to replace probably the BoB to make it work.
Do you have manufacturers documentation for both the BoB you already have and your stepper drivers?
I'm hoping there might be a simple solution.
If your budget is tight then you may have to consider making an amplifier solution....one IC...$4.00 or so
and 2 sq inch of vero board...not that hard.
Craig