It's pretty easy to do. Just remember adding more float switches adds more points of failure. Although, in this situation it's worth the risk - You will have a situation where a single failure can take your ATO offline, but it would require two failures to overflow your sump.
Instead of just telling you what to do, I'll try explaining it so you have a better understanding of the reasoning.
I'm sure you know how your existing ATO system works. The float switch activates the relay which activates the pump. Switches are either NO (Normally Open) or NC (Normally Closed), which means that in it's normal mounting position, when not being acted upon by an external stimulus the switch is either open circuit or closed circuit.
Now think about what you want to accomplish; You want the pump to run IF there's water in the reservoir, IF the sump isn't overfilled already and IF the sump level is low. You want the pump to stop as soon as any one of these conditions is not true, so you need to wire your three float switches in series.
Start at your reservoir; you're obviously going to mount it at the bottom. Remember what I said about NO vs NC? While the reservoir is full, water will be acting on the float, so you need your float to be open circuit when water is NOT acting on it, which means your reservoir float needs to be NO.
Next, look at your current ATO float. When water is low and NOT acting on the switch, you want to allow the pump to come on, so obviously you need your main float to be NC.
Your high level float is easy. You want to allow the pump to always run unless the water gets too high and acts on the float, so again, that would be a NC.
While you're wiring all that in, I'll make a suggestion. When doing maintenance on the tank, you'll sometimes do things that will bring the water level lower, but you don't necessarily want your ATO coming on. You can add a simple On/Off switch to your float switch chain that will stop your ATO from coming on without having to unplug it.
Limit switch logic is pretty simple and you can do a lot with it. The most important thing to remember is that when wiring switches in series, you are creating an AND circuit - the circuit only goes live when X AND Y AND Z are true. When wiring switches in parallel, you are creating an OR circuit - the circuit will go live if X OR Y OR Z is true.
