Hello,
like many things, you also can change many details of your Piano sound. The sustain is done at AMPENV, yes that´s right. But be aware: Since the pianos consist of many layers that respond to different values of velocity, you have to change every one of them. This might end up in a messed up sound, that does not really sound like it should be - you may have many hours of tweaking to do. The "natural" setting is a preset that can easily be used while programming a new sound. Usually it fits to the selected waveform, until you want to do something unusual with it.
There are different pianos inside the PC3 line, some have more "body" than others, check how its done there and maybe copy the relevant parameter settings.
Different approach: Sound engineers usually use compressors to even up sounds and also lift up decay phases. They are not longer, but they are louder as long they sound. Maybe that´s your way to go? But a comperssor will also level up soft volumes. But that´s not always a bad thing, since the sound still "feels" soft, it´s just a bit louder. Compressors a commonly used, often where you don´t know.

Hi KaffiMusic,
Thanks for the reply. I auditioned different piano presets, and noticed that the ones with compression do have longer sustain time. Like you said, I think editing the presets is a lot of work because of all the different layers

Try program 842 and 843. On 842 remove the compressor from the effects.