For ex, ADMIRAL is the normal vehicle, but I want to change admiral into police car so I can select admiral in car selection procedure at LCPDFR. How can I do that?

I don't think you can but, if you do that then just spawn it or do what I do.

 

Go on duty, hit " ` " on the keyboard(By the 1 above the letters) type in forceduty, hit enter so your now on duty then just spawn the admiral in the trainer. That's usually how I go on duty.

Well you can export the admiral using either spark iv or open iv (i recommend spark iv for extraction & open iv for other things). And after extracting it place it into a police slot (i.e. police2, noose, fbi, etc.) Now if your els is setup to use headlights, tail-lights, and indicators than you can have a decent unmarked police vehicle. If you want lightbars, you have to edit that admiral which is beyond my knowledge. 

 

open spark/open iv -->export vehicle (texture optional)-->place in a police slot--> for further work on new cop car youtube it ;)