Is it possible to use a single wire smoke alarm with an alarm system?
Such connections are not UL, CSA, or NFPA rated because the alarm panel cannot supervise the smoke alarm circuit.  2 and 4 wire low voltage alarm circuit are supervised and report more to the alarm panel than simply an alarm condition. For example, if properly wired, damaged wiring or missing detectors will cause a trouble condition.

You can physically do what your wondering (use the relay between 120v smoke alarms and an alarm panel).  Only an alarm condition can be detected (not low battery as the 120v detectors have).  

In a previous version of this answer I didn't say it the best, but as @gregmac points out in a comment, if you do this for your own benefit fine.  Don't ask your insurance company for the discount given for a listed monitored fire protection system, because this is not an approved method.

The smoke detector manufacturers such as Firex and BRK do make such relays available to activate other devices such as strobes or exit lighting (not to be confused with an "EXIT" light in a commercial or multi-family building), but not for means of UL or CSA rated alarm reporting.