If it is stainless anything, the chances of it being soft are slim to nil. This is because stainless steel is hardened with an air quench, so they cannot leave the tang soft simply by only dunking the blade in oil. Torching the tang might soften it a bit (temper) but I can't see anyone doing it commercially.
You can test it for yourself. Try filing the end of the tang. If it skates the file, it is full hard. If the file just barely bites, it is still hard. If the file bites like it does on a bit of mild steel, you are good to peen away. It it is hard, peening won't do you any good at all.
How long is the tang? Is it so long that it will poke out the back end of the handle you want to make?
If you are worried about 4+ inches of glued tang coming loose, you could get a carbide tipped "universal" drill and put a 4mm hole in the tang and then run a 1/8th pin through that.