The "most safe" and legitimate way to do it, is to go with both uni's to a uni place and show them what you want.
OR, you can do as mentioned and use the last bit of a S13 shaft on your KE column, but if your going to do this, I would sleeve the column so that you can get a weld on either shaft and not just "butted up".
OR you can do what I did in my first conversion, I used a KE/AE uni, on the rack side I drilled a 5mm hole opposite the "slit" in the uni, then I jammed a screw drver in the slit to spread it big enough to go onto the S13 rack, once on I used a longer bolt through the uni and place a nut on it for added tightness/strength. This probably would have been enough but I then tacked a small weld in the hole that I drilled earlier. Dodgy, yes I know, I have been through it may times in the SR/CA into AE/KE thread you can find on this site, but truth be told, I have had some larger hits than most people in my old car and still nothing ever broke.
Disclaimer: Do the top option.