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What type of detectors do you have?
Installing outlets for additional detectors on the second floor is done all the time. It's a matter of extending the circuit from one on the first floor or a basement. The compatibility of new and old depends on what you currently have. At worst the first floor units will need to be replaced. An alternative would be to use an interconnected battery detector and CO detector. Might be easier and cheaper to do that. I like those detectors. They don't depend on 120V circuit, the batteries last 2 yrs and they speak (fire bed room #1, smoke in living room, carbon monoxide % in kitchen, battery low). They can be installed over existing boxes and be moved when and if necessary.