South Korea is just a “global partner” and protections afforded by NATO article 5 do not extend into “global partners”, only to “full members”. So, from a NATO perspective, it would be very convenient to send South Korean troops (or from any other non-NATO country) to reinforce Ukraine and avoid triggering Putin’s claims of direct NATO involvement. In any case, South Korean cooperation would be needed to help handle an eventual influx of North Korean defectors and POWs