Can I splice wires in an old ceiling light box and should I add a ground conductor?
On this site there have been discussions from knowledgeable people about the importance of having every hot and its paired neutral conductor in the same cable or conduit. This is so the net time varying magnetic fields surrounding the wires are zero and so will not induce currents in surrounding metal. Induced currents cause heating of the metal.

I think you should find out what this pass through wire is being used for. This seems like a situation requiring a current clamp meter to find out the current (including the direction) in each wire under load. You may really need an expert electrician to look at this.