Adding a receptacle with two wire system, can I splice into an existing ground wire to ground my neew receptacle?

can I splice into an existing ground wire to ground my neew receptacle?
What does that mean? If you have a 2-wire system, meaning your cable is 2-wire without a ground, you do not have a ground wire to splice into.

If you question is "How do I replace a ungrounded receptaclel (two holes) with a grounded receptacle (three holes)". The answer is you replace it with a GFI receptacle. The GFI is not grounded and you mark it "Ungrounded".

This can be confusing, so I often don't mention this,

You can extend a 2 wire cable to a new receptacle, and then ground the new receptacle to the grounding electrode system, which means you run a green wire from the new outlet to either the ground rod, or the incoming water line, no farther thant five feet from point of entry into the building, or a few other locations, but not connect to any grounding conductor.

My stance is, if you can run a ground wire from a n ew receptacle, you can run a new feeder to the panel, and have the new outlet grounded properly with a new 3 wire cable.