I assume you have no experiance with software engineering. If this assumption is wrong, just hit the link below 
Software engineering is a very complex discipline involving many different skill sets. Know that you can teach yourself all you need to know to make this happen and make a clean contribution to the project. That said you need to understand that it will take a very large investment of your time. On the up side, you won’t just be getting that mod you’ve been wanting for your favorite game, you’ll also be learning a marketable job skill 
The repository for C:DDA can be found here: https://github.com/CleverRaven/Cataclysm-DDA. This is where all of the source code lives. Click on the file named “COMPILING-CMAKE.md” (assuming you are on a Windows OS) and start down the rabbit whole.
For everything that you don’t know what the heck it’s talking about, ask Google until you do.
If you do not want to make a very large investment of your time into learning these skills, try going to the GitHub page linked above and opening an issue requesting that a developer add your feature. Please explain in detail how it would work. If someone likes the idea and gets a free weekend, you might see it end up in the game!