I do not practice in PA, but the answer to that for VA is: absolutely. It would have to be entered into evidence in accordance with the court rules.
Disclaimer: This does not constitute legal advice and does not create an attorney client relationship. I practice law in Virginia and do not have direct knowledge of the laws of others states.
Almost certainly you can use it, and you should keep a copy of the support order handy to use for that purpose (although the Hearing Officer will have it handy at the hearing, anyway). Be aware, though, that the weight the court gives that determination is going to be affected by the other circumstances of the case. Frankly, I wouldn't recommend basing your case on the determination -- that would be a great mistake -- but suggest instead that you be prepared to support your contention of your opponent's earning capacity by other means. When it comes to proof I'm a firm believer in overkill, and coming in loaded for bear will help you counter your opponent's "that was then, this is now" counterargument.
I practice primarily in Allegheny County, and in my experience what happens at a support hearing has a lot to do with the attitude of the Hearing Officer... and you might not get the same one you had last time! So, don't plan to coast your way through the hearing just because things worked out well last time around.
Attorney Michael B. Greenstein
This response is offered for informational purposes only, does not create a lawyer/client relationship, and should not be taken as legal advice.