Depending on what you mean by "a specific pressure differential":
If you just want to drop/increase the pressure, regardless of flow rate, which is quite unlikely, you can just use a pressure source with a negative pressure increase.
If you want to find the orifice area needed to achieve a certain pressure drop by a given flow, like from a data sheet, you can set up a test harness with two ends of the restriction connected to reservoirs set at the specified pressure, then watch the flowrate. Adjust the restriction area so you get the right flowrate.
