Sure, given enough time and background knowledge. I myself developed 3 custom platforms: For Gigadevice GD32 (ARM) chips, Winner Micro W600 WiFi chips and Boufallo Labs BL60x chips. I also contributed to lots of other platforms. All one needs is a good reference project (an MCUXpresso project in your case with all verbose compiler invocations) and an understanding of how PlatformIO works (packages, builder scripts, platforms, board JSON files, …) to do it. See e.g. Arduino Due (or other SAM3/4) upload fails with ATMEL ICE JTAG on VSCode with PlatformIO - #8 by maxgerhardt.