is it possible to get cop skin example(uniform) that i download? in FiveReborn and siren to cop cars ?
You can if you have trainer that Fivereborn supports and you can add siren to cop cars with trainer too.
If you want to make a server where everyone spawns with cop skin i cant help you. 
I just replied at http://fivereborn.com/topic/443/skins Maybe that will help you somehow tho i did not try anything by myself. Searched for spawnmanager in server i downloaded from release topic.