Can the analog inputs be scaled in VFD settings? What range of VFD frequencies will you be using?
Namely, if you can setup VFD in such a way that 3.3 V on the frequency reference input means maximum spindle RPM that you need, then you only need to connect an RC filter (resistor and capacitor) to PoKeys output and connect the output of the filter directly to VFD.
If you really need 0-5V range, then you can put a 74HCT125 line driver (or some level converter IC), connected to 5V supply. This will convert 3.3V digital signals to 5V digital signals - you will need to put an RC filter to the output of this IC and the output of the filter to VFD.