I was wondering if it was possible to set the siren settings for different police slots i.e. (Police, police2, sheriff etc) like to change the police car siren to say the sheriff SUV slot, and the police 3 to motorcycle?

Sirens are not separately defined, so there's no way to change a cars siren without changing all the sounds the vehicle uses.

Yes. When you go to the vehicles.meta for a specific vehicle, you will see a line that says

<audioNameHash>police</audioNameHash>      This is from the standard police slot.

 

For police bike, it should say <audioNameHash>policeb</audioNameHash>

 

You can just change the value to whatever siren you want. All of mine have been changed to use the touchmaster delta siren I have installed for the regular police slot, so they all have that "police" in there.like the first example.