Pumps pump liquids. Compressors compress gas. I seriously doubt if you will find a piston pump that will compress natural gas. The liquid head produced by a pump might be used to compress a gas downstream, but the pump itself can't compress gas.
Can you explain the details of your design? This will be a revolutionary first.
You basically want a PD type compressor like a recip. You won't do it in one or even 2 or 3 stages though because of the heat of compression that occurs. The number of stages will depend on the low pressure conditions and the type of gas (some gases have lower limits on the maximum temperature they can accept.
