Arduino offers the possibility to create a custom board support based on an other core:Referencing another core, variant or tool.

@RickKimball provides a great use case based on the STM32 core:

As he mentionedhere:

Just be warned I haven't tried that GitHub since you added the BluePill as an official board. However, it does give hints at some of the things they might need to do. Just don't expect me to answer questions on why someone's personal board doesn't work. : )

https://github.com/RickKimball/vendor