First of all, download the files for the vehicles you'd like to import into the game
Once you gathered everything you wanted, you will need to get a DMO file, a glitched game save file to import your vehicles, you can find one easily online.

Open the DMO file in the gta 5 save editor program, as for the selected character stay on Franklin, then go to garage -> vehicles and select a vehicle you'd want to replace. Finally next to the search bar, you'll find a button "replace vehicle", click on it and it will open a window where you can select the .GTAVV file and it will replace the selected vehicle by the one you choose. It's possible tp do several vehicles in one save but i don't recommend doing more than 7 at once.

You can then save the changes and exit save editor, then head over to edit the save to match your account, otherwise you won't be able to copy it on your ps4. If you don't know how to do that, I recommend using a discord bot as it is a free method and you only need a discord account, just join a server with one of these bots that can edit your save and carefully read the instructions. If you have save wizard you probably already know how to resign the save. Finally plug the usb stick in your ps4 and head up to the settings and copy the data from your usb stick to the ps4 storage. Launch the game and you should have the imported vehicles in your garages.