Taking existing files and modifying them is definitely not recommended, since the moment we release a new version of the Community Pack, your custom versions will be overwritten.
Mergon has done some early work on Saltmarsh already,
here,
here, and
here. If you grab those files and put them into your HL data folder for 5e, they'll get you part of the way.
We haven't tackled how to handle the
existing backgrounds that get Saltmarsh-specific elements yet. It's complicated, but the solutions would to either create an adjustment that says "this character is using the Saltmarsh background variants, change background X to that", OR to create new copies of the backgrounds and include the Saltmarsh info.
If you do play around with it and come up with something, feel free to share! We'd definitely want to include it in the pack.