I'm installing a light fixture in a room that was built onto the outside of a cinderblock wall house. Getting power must go through the attic (unless I wire mould up the wall from a receptacle: don't want to) and then be fed onto the ceiling (with wire mould). I've already fed Romex from the switch box in the kitchen through the attic, but it's a hot switch line only... so I have hot to the new light but no common. My question is this: can I splice into a Romex line in the attic and use the common (ANY common) for the new light. I guess what I'm trying to confirm (what I believe) is: are all common wires "the same" and so any common (white) wire would serve to complete the circuit and power the light??

No, all Neutral wires are not the same and you should not pick up the power from the switch.

Pick up the power from an outlet where you can splice onto the Neutral, Hot and Ground.

Keep them all in the same circuit, do not pull a hot from 1 line and a neutral from another.

Is there anyway to use the circuit in the kitchen switch box (no common visible) to run this light? I just can't dfo wire mould from the receptacle with a surface mount switch box..?

If you have a switch loop at the kitchen light switch, power must be provided to the kitchen light fixture itself. Just pull you cable through the attic from there.