The left channel output on a hardware synth will generally be set up (and labeled) as a mono out when nothing is plugged into the right channel, so there's no need to merge stereo from the synth to the mono input if that's the way you decide to go.

However, I would strongly advocate for recording in stereo from the Kurzweil. Aside from the fact that some instruments like Piano will have been sampled in stereo, and drum sets will have panning built in, a lot of patches will have stereo FX like chorus, phasing/flanging, multitap delays, Lesle simulations, etc. that are an integral part of the sound and that you'll want to be able to capture to take advantage of all the work that sound designers did.