Adding a single light switch from a 3 way light
Short answer you can't.

If what you are describing is correct you only have three conductors in the box attached to the light fixture. That would be a ground, neutral and a switch leg from the 3-way switch. So you do not have a source wire in that box only a switch leg. Your best option to install a separate switched circuit for the second light fixture and run a completely new switch and switch leg.

Hope this helps and good luck.