If you have a hot shot co pilot paired with the R7 astromech, could you force an enemy ship that you have target locked to spend it's focus, then spend the target lock and force a re roll, thus making your opponent waste their tokens?

And does this combo only apply to Norra or Shara?

No. Firstly, you play your mods before the attacker, so you'd have to use R7 before your opponent would have the chance to focus up.

Second, you don't decide when or how the attacker would spend that focus. He doesn't even have to focus up if he has something else he can spend a focus on, and he could spend it as the first mod, or the last, or anywhere inbetween.

Attacker rolls attack dice -> Defender mods attacks dice -> attacker mods attack dice.

Cool, thank you. Glad I didn't try and run a broken setup

You could, however, see that he rolled 3 focuses and knowing he has to spend a token, re-roll them to try and make that spending of the token less good for him (by coming up blank the second time)