I cannot see it being able to be done. Theres just not enough wires. Basically whats missing is after you get your switched power to your light you need a path back to the main (source) neutral. Or any neutral really. You do have white wires in the 3 way switch boxed but they are acting as travellers and not neutrals. Sorry for your bum luck. I hate being short wires!
If you could pull a 3 wire to replace the 2 wire between the source power light fixture and the 3 way switch box on the right, than you could then feed a light fixture off that 3 way. I am not sure if thats easier for you...
This is an unmitigated disaster
Firstly, 60A auxiliary heater circuits are normal enough, but they don't have neutral. The wires are L1-L2-ground or black-red-bare. There is no way to get 120V out of that circuit, except by bootlegging neutral.
Bootlegging neutrl means misusing ground as neutral, intentionally connecting a "hot" wire (via a light bulb or tool) to the ground. If the ground-neutral bond in your panel has any sort of problem, it will electrify every ground in your house. Bad idea.
Nothing prevents someone from plugging in a power strip and four 1500W heaters. That will overheat the wiring and start a fire.
This is terrible work!
Must be more going on
Thing with heat pumps is there are usually other circuits involved. I am surprised that the air handler would share the 60A aux heat circuit, I would expect there to be a 15-20A circuit there for the air handler. I would think it might also be 120V. If so, it may be possible to borrow it for the attic light. The attic light must be on a 15 or 20A 120V circuit. It cannot use part of a 120/240 circuit.
The total of all hard-wired loads on a circuit must be less than 80% of total circuit capacity.
You cannot have any receptacles on a circuit unless the hard-wired loads are less than 50% of total circuit capacity. So you may need to change lamp fixtures.
Or, go 24V lighting
In this day and age, LEDs are so efficient that 5W of lighting is a fair bit, and two would light up an attic decently enough. Most furnaces have a 24 volt AC, 40VA (similar to watts) transformer with surplus capacity. You could grab 24VAC off the transformer and take it to LED lights. You would need to find LED fixtures that have no replaceable bulb, and work a range of voltages including 24V AC. You can use common thermostat wire to hook them up. Aim for an even number of lights, and hook each one reverse polarity of the next. Sometimes LEDs will tolerate AC but only light on half the AC cycle, by reversing polarity to every other one, you assure the full cycle is used.
Best Answer
Nope
NEC 210.23(C) limits 40A and 50A multioutlet branch circuits in dwelling units to powering fastened in place (i.e. built-in) cooking appliances, not freestanding ones. (A regular range circuit only has a single outlet, so it is governed by NEC 210.22 instead.)
Your 40A branch circuit could power either range, though, as a 13kW nameplate range comes out as 8.4kW of demand-factored load when you apply Table 220.55, note 1 to it, and NEC 422.10(A), paragraph 4 expressly permits the table 220.55 branch factors to be applied to household cooking appliance circuits.