As far as I know, new mutation that increase charater’s maximum carry-able volume can be added by compiling cataclysm. Then, How can I compile Cataclysm.
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!
I’m using Code Blocks myself to compile my own modifications to the game’s internal workings (The last thing I tinkered with was a “Disable bullet animations” option cause my vehicle turrets firing a 60 burst for half a minute every turn was driving me insane and I didn’t want to disable the animation system entirely). I also use Sourcetree to keep my modifications across the latest official changes by just branching off for my own stuff and merging locally when new changes are made.
For Code Blocks I followed these instructions: http://smf.cataclysmdda.com/index.php?topic=7646.0
It’s Robik’s reply, post #5. After I got everything ready, Code Blocks still threw out some errors but it wasn’t difficult correcting the problems by reading the build message log and making the appropriate changes.
For Sourcetree or whatever Git client you choose, you really just need to invest a bit of time going through a Git tutorial and become familiar with all the terms, then read whatever tutorials come with your Git client. Going in head first blind with Git isn’t a good idea.