I posted in 2017 or 2018 a thread about me re-using a pin in an 11 speed chain. Virtually everyone thought I done a terrible thing, an un-safe thing or that it would blow up and I'd be sorry.
Turns out that they were incorrect. My chain lasted till 2020 when I changed it out for wear (stretch) after at least 4800 miles of use. Unfortunately I didn't track it well however I think it actually has a few thousand more miles due to rides I didn't upload.
I personally wouldn't extend my chain unless I had the links that I know are of the same chain. If you have them just use quick links to fasten them in or if you have the Shimano pins that you break off the tapered end after pinning the chain, those are good too. But probably only for Shimano chains they are made for.
It was a lot of trouble for me to get that tiny piece of a pin aligned correctly to push in. I wouldn't have tried so hard at that if it wasn't for it being pretty weekend with the shops closed and me wanting to ride.