Retrofit ground wire from water main ground
Yes, you are allowed to retrofit ground for a range, and that is top of the list of important ones to retrofit.
You can indeed  retrofit ground to the Grounding Electrode Conductor, which is the (typically bare) copper wire that runs between the service panel and the grounding rods/water pipe.
The typical way to do that splice is with a &quot;split bolt&quot; which is a bolt-and-nut with a slot cut down the bolt; you put it around both wires and then run down the nut to whatever the instructions say.  Tight enough to clamp, not tight enough to shear the GEC clean off (that would really, really suck because you'd have to run a whole new one; splices not allowed in a GEC).
I would give the GEC a spit-shine and polish with a bit of steel wool or green Scotchbrite pad, so the surface it'll contact is shiny. Don't use it for food afterwards, that green copper oxide is not super healthy.