It is the heat from the light fixture that you have to worry about, not the heat from the current flowing in the wires (assuming the wires are sized large enough). See 410.67 for recessed fixtures, you can slice the wires in an outlet box at least one foot away from the fixture. Also see 410.11 and 410.33. At any rate, you shouldn't have to run new wire back to the breaker box.
