@Paul_Phelan
What brand is your old (dumb) alarm system? The DSC PowerSeries can be linked to SmartThings and the wired external siren still used by using the Envisalink module. It is however likely the siren would only be triggered by a sensor of that alarm system rather than a SmartThings sensor. However you can arm/disarm it via SmartThings.
See DSC + Envisalink 4 + smartthings-dsc-alarm-server amongst other threads.
By the way, I share your annoyance that all the new so-called smart alarm systems do not offer external sirens or at best only battery ones. This applies even to the unavailable in the UK ADT/SmartThings smart alarm system. You would think a company like ADT would know better.