In this repo you will find aditional core devs for stm32duino. https://github.com/stm32duino/Arduino_Core_STM32
How to add a new variant to the stm32duino Project: Copy the corresponding folder to the stm32duino "variants" folder in your Arduino Folder, then open "boards.txt" in your "Arduino15\packages\STM32\hardware\stm32\x.x.x" folder and paste the corresponding segment from the BOARD_DEFINITIONS folder on my GitHub to the right part of the file with the correct STM32 CPU Series.
There are also PlatformIO Definitions
If you have any questions just write a Issue.