Doubt it. I considered myself a dab hand at joining chains back in the days of 9sp, I did loads – and then I lengthened an 11p chain and despite applying all my skill and experience I couldn’t get it to stay together. There just isn’t enough pin to work with, and I think the ends are flattened/riveted (this was a KMC chain) which then either widens the hole or breaks off the lip when you try and push the pin out. I’ve not seen a 12sp chain but I’d imagine it’d be even more difficult.
You could do it with another quick link though. That’s what I did, I now have two in my chain.