1. Edit the vehicles.meta inside the f450plat folder.
2. Find the section that begins with <trailers> and ends with </trailers>.
3. Find the trailer you would like to add from the desired creator. 
4. Create a new line and copy and paste the new trailer line into it. 
Basic example- 

         <trailers>
        <Item>TRAILERONE</Item>
       </trailers>

Add line to make it-

         <trailers>
        <Item>TRAILERONE</Item>
        <Item>CREATORTRAILER</Item>
       </trailers>

5. Save and import vehicle.meta back into OpenIV. 