You need GCFScape[nemstools.github.io] to unpack .vpk files.
Next you need to create or download the mod that you want to appear on certain campaigns. For example, I'll download a SIG Sauer P220R & Glock 19 mod for pistols.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2123437464
Subscribe to the mod, launch Left 4 Dead 2 and close it when the download finish.
After that, go to your Left 4 Dead 2 folder and locate the mod that you downloaded from workshop.

Open a .vpk file with GCFScape and extract all files to any empty folder. You can just create a new folder on your desktop or somewhere else.

Go to the Left 4 Dead 2 folder and find update.

In the update you will see a pak01_dir.vpk file.

Next open the missions folder.

This is our main folder for making mods appear on specific campaigns. You will find there almost every .txt file for every official map.

• campaign1.txt - Dead Center
  
• campaign2.txt - Dark Carnival
  
• campaign3.txt - Swamp Fever
  
• campaign4.txt - Hard Rain
  
• campaign5.txt - The Parish
  
• campaign6.txt - The Passing
  
• campaign7.txt - The Sacrifice
  
• campaign8.txt - No Mercy
  
• campaign9.txt - Crash Course
  
• campaign10.txt - Death Toll
  
• campaign11.txt - Dead Air
  
• campaign12.txt - Blood Harvest
  
• campaign13.txt - Cold Stream
  
• campaign14.txt - The Last Stand

To get extra .txt files go to left4dead2 and find the same pak01_dir.vpk file with missions folder.

• credits.txt - Credits in extra menu (Not very important)
  
• holdoutchallenge.txt - Holdout mutation with L4D1 survivors on Swamp Fever and Hard Rain
  
• holdouttraining.txt - Holdout mutation with L4D1 survivors on Death Toll
  
• parishdash.txt - Dash mutation with L4D1 survivors on The Parish
  
• shootzones.txt - Shootzones mutation with L4D1 survivors on Dark Carnival

Select the campaigns that you want and extract them in missions folder of your mod.
Should look like this:

For custom campaigns you need to download the campaign first and unpack it. (Look in first section how to do it)
Extract the .txt file of the campaign to your missions folder in your mod. For example I used Devil Mountain
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=316053033

All that's left to do is to pack a .vpk file.
To do this you need to go back to the Left 4 Dead 2 folder, then to bin folder. There you will find a vpk.exe file.

Drag your mod folder into this file. It will create a .vpk file of your mod.
Now you can place it in your addons folder or upload it to the workshop.
Just remember to credit original mod authors if you editing a mod from workshop.

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2863319780
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2863314323
You can make a campaign specific mod with files like:

• Models
  
• Textures
  
• Sounds
  
• HUD/UI
  
• Animations