I'm not at all familiar with the Roland XP-80, but looking at your question from a generic viewpoint, one way a "sound designer" will try to fatten up a sound is to use two oscillators but detune them slightly. On a "ROMpler" workstation keyboard (i.e., one that plays sampled sounds, as opposed to a synthesizer), this same sort of idea can be accomplished by applying the Chorus effect.
You could also try adjusting the Filter Resonance and Filter Cutoff settings if the keyboard has them, or adjusting the equalizer settings as desired (e.g., to increase the lower frequencies and decrease the higher frequencies).
Another way to fatten up a sound is to use two oscillators tuned an octave apart. On a workstation keyboard, this can be accomplished by using two layers set to the same sound, but with one layer set an octave higher or lower than the other layer. Of course, you could also layer two different sounds together.
And you could combine these different approaches-- layer two sounds together (either the same sound or different sounds), set them an octave or more apart, apply different Chorus settings to each layer, as well as different filter settings. Or you could try different envelope settings-- e.g., the primary layer could have a faster attack followed by a moderate decay, and the secondary layer could have a slow attacker followed by a long decay/sustain, such that the primary layer is emphasized during the first part of the note, but the secondary layer is more dominant after the initial attack/decay phase.