Reusing system components is a common question and certainly a good idea to limit up front costs. If you have a lot of sensors a wired to wireless translator will be very cost effective.
I would recommend testing and trying to reuse all intrusion alarm sensors, however keep in mind existing keypads cannot be reused, and always check life safety devices like Smoke Detectors for their expiration date, and expect to replace them if the system is very old.
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There are two hardwired-to-wireless translator options for the standard 319.5 mhz model of the panel.
PowerG Hardwire Module. (PowerG RF, supports 18 2-wire smokes)
IQ Hardwire 16F. (319.5 RF, supports 10 2-wire smokes)
The Hardwire 16F specifically states to use one of the following models of smokes: System Sensor 2W-B, 2WT-B, or 2WTA-B
Do not mix and match wired smoke detector models, use all the same model.
Are the siren speakers re-usable?
Maybe. Are they sirens or are they speakers?
A piezo siren can be reused, though depending on the power draw of your model you may need a separate relay and power supply. The panel and modules support lower amperage sirens.
The siren outputs are relays, not speaker drivers. If you have speakers there would be no driver for it.
The panel has a built in 85 db siren as well.