That would be brilliant, I bet gabester can make some awesome fictional siren with his music skills. You also should have different siren modes, if you look into real policecars, you can see they actually are able to use the horn with the siren on, and have another priority sound too.
Maybe the way you can make it is to add the sirens and add the lights to the GMPE and make it so you can edit the siren sounds in some file and lights are used by pressing "I" might be useful as well to update the police texture so theres more to edit apart from the [POLICE, CALL911, 62] & the badge so full body decals.