As Ed Beal points out, there is no way to wire a new light from the right hand location. If you use the neutral from the right hand location and the get the power from the right hand switch there are two problems:
1) You will create inductive heating of any metal parts on that circuit since the feed power and return power are not in the same cable. This is dangerous and could cause a fire.
2) The right hand box does not have the switched wire that is in the left hand box. It only has the hot feed and the travelers. So, you could only wire the light always hot or miswire it to one of the travelers. So it would always be on or on only when the left hand switch is in one of its two positions.
So, in the right hand box, you could replace the 3-way switch with a duplex switch that has a 3-way and a single pole switch. Rewire the 3-way the same. Then use the extra single pole switch for the new light with power and neutral from the other cable.
Or, you can run som new cable from the light or the right hand box. Wire can usually be fished into walls (look for videos on YouTube) and you don't have to remove any drywall.