Hi, I'm really glad you're enjoying the mods.
Here's a guide on addons- unfortunately, it's incomplete, but it may still help some: https://github.com/offyerrocker/PD2-AdvancedCrosshairs/wiki/Creating-Advanced-Addons

I'm glad you have some experience and initiative with modding. I hope you like a challenge, because this will be a bit more difficult than editing Source HUD layout files.
Fortunately, you don't need to really write new code, just copy+paste some existing code and change it a little. (And make sure it's written correctly for Lua syntax.)

As for how to make your changes, here is what you have to do:

1. Add more parts for the double-bar that reflects your armor. The code that defines their initial placement is in the parts table in the addon.lua file. There, you will find each of the existing parts, so find the ones you want to copy (the empty bar and the filled bar are two separate images, so you will probably need to copy two parts- the empty and the fill.) and paste those new entries to the parts list, at the end so that you don't mess up the order of the existing ones.
2. Adjust the distance value for your new entries only, because otherwise they'll overlap with the health bar part. Tweak the value until it's to your liking, but I'm guessing you will probably want it to be somewhere around 80.
3. Add the code that makes it reflect the armor value. You'll see some code that says if index == 1 then, with numbers going up to elseif index == 4 then. This code controls scaling or movement for each part, and index tells the code which part that code is changing. Since you added new parts to the end of the list, they should be 5 and 6. Copy the code for the left outline and left fill, and change which number it's checking the index for. Then, you need to change it to reflect the armor instead of the health. I made a "shortcut" to the health ratio as health_ratio, which you can find around line 33. Copy that line and replace health_ratio with armor_ratio on that line, as that is what the function is called. Replace health_ratio with armor_ratio in your part code as well.

Getting the Lua syntax will be a challenge for a novice, so I recommend looking at some beginner's guides to Lua, or asking someone for help in the likely event that you don't get it perfectly right the first time, but I think you can do it.