Using Existing Modifications as DLC Packs
Firstly you'll want to fetch the keys for ArchiveFix (afix) to do so download and unpack the ArchiveFix package.
Open Command Prompt and type (replace with where you unpacked ArchiveFix)
then open GTA 5 and let it sit on the loading screens, return to your command prompt and enter
this may take some time to fetch the encryption keys, as it depends on your CPU power. Just be patient and let it do its job.
After you've successfully acquired all the encryption keys, we can move on.
Afixing and using Add-on vehicles
We'll start by finding a modification from here.
Once you have found your modification of choice (This tutorial will be using this: https://www.gta5-mods.com/vehicles/mazda-rx7-c-west)
Inside the archive (/Add-on/) there is a file called readme.txt — inside it references the modification as
So we won't append dlc_* to our folder name.
Navigate to your ArchiveFix root directory and create a new folder with the name of the modification (rx7cwest) and inside create the following folders:
Move the dlc.rpf from the downloaded mods archive to the newly created "oldrpf" folder.
Using OpenIV navigate to the /rx7cwest/ folder inside our ArchiveFix root, making sure you're in EDIT MODE right click the dlc.rpf and 'Save Contents/Export' select the /unpack/ folder we just created.
Navigate inside /unpack/ copy all the unpacked folders into /newrpf/.
Drag and drop any subsequent *.rpf files inside /newrpf/ onto ArchiveFix.exe
Using OpenIV create a new dlc.rpf file inside /newrpf/ and drag all the afixed files from inside /newrpf/ (make sure you don't copy the new dlc.rpf file) into the dlc.rpf inside OpenIV
Drag the new dlc.rpf file onto ArchiveFix.exe to encrypt it
Create a new folder inside RageMP's /server-files/client_packages/dlcpacks/ called /rx7cwest/ (remember the actual add-on's name dictates the name of this folder) and drag the new dlc.rpf inside.
Making use of the new mod
Start your server, and connect to it you will download the new /dlcpacks/ files, and then once it reconnects automatically close the game.
Reconnect to the server and spawn the new vehicle by typing the hashname (if you're not sure, you can check the setup2.xml file inside dlc.rpf) in this case it is "rx7cwest"