Negative PP is achieved through an oversight of the PP system. The PP is actually a curve that goes down after a certain point of insane performance (which you should never be able to reach through normal means). In some high-power hybirds with positive PP (The 50000HP R8 being one of them if memory serves right), you will notice that acts that normally should increase PP (e.g. adding downforce) will instead decrease PP, with PP-decreasing acts (e.g. limiting power) increasing PP. The negative PP X2010 is an extension of this, its performance being so insanely high that it's well into the negatives.
As for non-X2010s to reach negative PP, yes, it's doable, but most (if not all) candidates can only reach such PP through acts that will get the horsepower into the millions, but completely messing up the car's power band in the process (with the power band looking like the 15000HP NSX circulating out there). These cars are utterly unusable.
Oh yes, a little loophole abuse on the question. You can actually safely make a negative PP X2011 that's drivable, but from my experience it doesn't handle as well as an X2010 that underwent the same treatment.