Can I add a single pole switch to an existing thee way switch with same power source
It is simple and straightforward in this case. From the switch on the left take the hot neutral and ground to the new switch location. Then feed the hot (black) to the switch.
Now take the black to the other side of the switch with the neutral and ground to the new light location.
Install the switch and the black at the light will be the switched hot from the new switch you added.
This is the only location you can add with the diagram you referenced. If the hot and neutral are at the light, not the switched position, it will take 12-3 or 14-3 wire to do it, but it can not be done from the switch on the right.