Recently, i have added a police car to a non-police slot. I know how to enable ELS for the car, but is it possible to enable sirens and how?

Maybe this will help: http://www.lcpdfr.com/page/tutorials/index.action/_/vehicle-modification-tutorial/how-to-enable-sirens-and-lights-on-vehicles-that-are-not-in-an-emergency-vehicle-slot-r64

Ok, but thats not exactly what i meant. I want the sirens to be enabled and have other vehicles pull over.

Ok, but thats not exactly what i meant. I want the sirens to be enabled and have other vehicles pull over.

That's not possible. LCPDFR will only work with police cars placed in the available police car slots. Placing a police car model in a non-police car slot will mark it as just that; Not a police car.