Rather than having some kind of extension or just make it a unit to add to any fire station, maybe we should discuss the possibility of a Fire Marshall/Investigation/Arsons Office. You don't see a Fire Investigator in every fire station so maybe this could be an option. Everybody calls it's something different so that's why I put those three names up there.
So you could have up to three or four slots per building for these units. While I'm on the subject this unit could be called in after a fire or building collapse of some sort which could also give the player more credits for sending this unit to respond.
Similar to sending an ambulance to the right Hospital for extra credits but instead the player has the option to send or not to send a Fire Investigation Unit whether it is available or not. Again sending this unit would earn a little more credits to the call completion. If the player did not send this unit then they would just get the credits for said call.
If anybody else has another idea they would like to add to this please feel free to do so!
I don't want to see a fire Investigator Unit acts like a Battalion Chief or be useless. This would have to have some type of special use for any kind of fire or structure related call.