If you mean the sustain that you get by pressing the sustain pedal, there's no way to increase the basic length of the sustain, as it's primarily determined by the sound sample. However, if you crank up the reverb level of the voice and/or increase the release time of the notes then you might be able to increase the sustain time-- except the DGX-230/YPG-235 doesn't have panel controls or menu functions for setting the release time, so you'd need to change it via MIDI. If you have an iPad or iPhone, Yamaha has a free "Sound Controller" app that lets you change various sound parameters, including the attack and release times. When you search for it in the Apple App Store it doesn't show up under the "iPad Only" apps, just under the "iPhone Only" apps, since its screen display is optimized for iPhones-- but it works equally well on an iPad. (I honestly don't know why Apple doesn't have an "iPad or iPhone" option on the App Store's search screen.)
