Sure, why not. People add boards to the EarlePhilhower core all the time. All board definitions are conveniently auto generated, so you basically PR your change in that script plus the variant files that define the pins for your board.
https://github.com/earlephilhower/arduino-pico/pull/1828 is a good PR to look at.
If you don’t yet have custom hardware though, try to stick with the boards that are already there.
This question also largely overlaps what has already been asked at https://community.platformio.org/t/rp2040-custom-board-in-maxgerhardt-platform-raspberrypi/35845.