I just went through this and you can do it, pretty easy if the cars are already installed. Go LSPDFR>data>agency.xml and edit it. You can copy and paste the name of the vehicle(s) you want ai to patrol/respond for backup in for each agency.
Here's how mine looks for example for LSPD:
<Name>Patrol Unit</Name>
<Vehicles>
<Vehicle>police</Vehicle>
<Vehicle>police2</Vehicle>
<Vehicle>police3</Vehicle>
<Vehicle>police4</Vehicle>
<Vehicle>police5</Vehicle>
<Vehicle>police6</Vehicle>
<Vehicle>police7</Vehicle>