@intwolt, my apologies for taking so long to reply, but I was involved in traveling. Now that I've arrived, I can catch my breath and answer questions.
I'd just suggest searching online for the model number I mentioned. I'd recommend finding an actual alarm systems parts dealer, and not eBay. Do NOT buy
used smoke sensors: they have a definite lifespan after they're unpacked from their sealed packaging, even if they aren't connected up and "working".
If your existing smoke sensors are the originals from 20 years ago, they're long overdue to be changed. Maximum lifespan they can be considered reliable is about 10 years.
I think you can wire a reversing relay into the loop--TBH, I've never done it, because for a home system, if you can't hear the siren sounding, it's probably because you have an external siren and not internal, and a home big enough that you don't hear the external siren that well at one end of the house.
A better internal noisemaker in the vast majority of homes is to add an internal siren, centrally located, that will get a hearing-impaired person's attention even better than all smokes sounding, no matter where she is in the house. The wiring's a lot simpler too. Depending on how deaf she is, you might also think about adding an internal strobe light to the same "Bell" output as the siren.
One caveat: You need to make sure you know if you have (a)
Siren(s) or (a)
Speaker(s) as your sounder(s). A
siren is a combination of a
driver and a
speaker. Some systems have self-contained sirens, which are just speakers with the drivers built into the same housing. Other systems use a driver module in the control box, that drives the speaker(s) mounted outside or wherever you've put the sounders.
If you're currently using self-contained siren(s), then as long as you don't exceed 700 mA total current, you can just add another internal siren to the "Bell" output: Observe polarity.
If you're currently using an independent driver to drive speaker(s), then you need to add any new speaker to the output of your driver (no polarity). BUT, be sure the total resistance of all the speakers isn't less than what the driver is rated for. Most drivers are rated for a minimum of 4
Ω, so f8or example if you have two 8
Ω speakers, you can wire them into parallel to the driver output, giving a total resistance of 4
Ω.
And that's as far as I'm going to go into that IF-THEN tree until I know more about what you're working with, and how you want to go about it.