Think we need clarification of OPs needs. He doesn't mention spindle in OP only control of speed of a servo. Everybody's made the assumption of spindle speed control by virtue of mention of VFD but VFD is not a servo thing. If using step/direction controls then speed is related to step rate, so its not clear what analogue voltage is doing.

Creating -10 -> +10v from step (PWM) and direction (polarity) isn't hard but you need some skills in design, electronics assembly and testing. Jonathan has given part of a solution but its of little use without the skill/knowledge to do the rest.

I may be wrong but I know of no BOB that does -10v out... the speed output is invariably 0 -> +10v and direction goes to fwd/rev input on VFD.

@vre: please be more specific in your requirements; are you referring to motion control or to spindle speed/direction control?