You are going to need some external circuitry. An amplifier and level shifter can change the 0-5 V signal to a 2-10 V (or 10-2 V) signal to match the valve controller. This is a straightforward (but not trivial) circuit design problem. The data sheet you posted does not specify the input impedance for the control input, but it is probably high enough that the circuits could be built with standard op amps. The other option would be to find a DAQ device with a 10 V analog output.
This is not something which can be done in software.
According to the NI 6251 specifications the analog output can produce a maximum current of 5 mA. So you will need some kind of amplifier/level shifter/current source circuit as an interface between your DAQ device and the valve.
I suggest that you talk to your NI rep and ask about a 4-20 mA output device.
You are getting beyond the point where help over the Forum is working. Perhaps you need to hire a consultant to either give you the guidance you need or to do the parts of the job which you cannot do for yourself (at least within the time frame required).
I'm too lazy to design, build, test, & calibrate circuits that convert process signals between voltage and current and prefer off the shelf solutions. There are numerous Industrial Automation components that will make the conversion; I've used this one in several applications
