If you just have an updated version of the energia framework which platform-timsp430 already supports building for, then you can just use a platform_packages instruction in the platformio.ini to override the source for
aka framework-energiamsp430.
platform_packages =
framework-energiamsp340 @ https://github.com/youruser/yourcoolfork.git
Be aware that the source you point it at (e.g., a git repository) needs to have a package.json in it. You can copy the current one in <home folder>/.platformio/packages/framework-energiamsp430 and adapt the version field accordingly.
Add a new custom framework (that does not build on anything else) requires the appropriate declarations in the platform.json (which declares where to get the framework files, packaged as a “package” and where the builder script is within the platform) and an appropriate builder script.