Yes. If you adapted makeboards.py in your local fork correctly and executed it, then it spit out a new say my_custom_rp2040_board.json file. I’ll also assume you have a working-copy of your Arduino-Pico fork (that was recursively cloned with git clone --recursive otherwise you’re missing pico-sdk and tinyusb etc. – or git submodule update --init --recursive) on your harddisk.
Create any new PlatformIO project (e.g. Board “Arduino Uno”, Framework “Arduino”). Create a new boards folder at the same level at the platformio.ini. Put the generated board JSON file in there. You can then use a platformio.ini such as
[env:pico]
platform = https://github.com/maxgerhardt/platform-raspberrypi.git
; filename (without extension) of new board JSON in boards folder
board = my_custom_rp2040_board
framework = arduino
board_build.filesystem_size = 0.5m
; use local copy of forked core
platform_packages =
framework-arduinopico@symlink://C:/Users/MyUser/Desktop/arduino-pico
All of that would have also been documented in the Arduino-Pico docs and PlatformIO docs.