It's possible, I've added an add-on police vehicle yesterday for the 1st time and it uses ELS as well. By creating a DLC add-on for the non replacement vehicles you'll need to create a custom vehicles.meta, handling.meta and carvariations.meta and edit your dispatch.meta in your update\update.rpf folder to include the add-on vehicles used. If using ELS, make sure the name matches the vehicle name... for ex. police6