Jeff, You can use your existing driver cards (if this is the type I am thinking) the orange and brown wire are step and direction, I forgot which is which. Black and red are the power lines. I converted one for the high school here back 8 years ago. Beware of noise from the spindle DC controller board, I had to put some small caps on the step and direction lines or every time I ran the spindle the machine would move on its own.
a lot of the breakout boards can be scaled for something less that 10v. But I don't think it would matter, you could make a simple voltage divider of 2 resistors of equal resistance across the output of your breakout board and tap into the center for your output to your motor controller.