The easiest way to adjust stereo width is probably to do it in the DAW -- for example, Logic's Stereo Pan knob allows for this, and Pro Tools' dual pan knobs [in stereo channel strips] do as well. Either can narrow the stereo width, or even pan a stereo image partially to one side, like a stereo piano that occupies the stereo soundfield from full left to center, for example. [I'd imagine most other modern DAWs have similar stereo panning capabilities]
Options seem to be more limited if you're using the standalone version of Keyscape -- I don't have it here, but going strictly by the manual it would seem that there's only a stereo or mono switch, rather than a continuous width control..? The manual does refer to a stereo width control knob in one patch, but it's not clear if that's available for any of the other keyboards..