Since the Particle Mesh devices use the Adafruit Feather form factor, you may be able to use or adapt a Feather device, e.g., the Adafruit Feather HUZZAH. Looks like it can be found in the official KiCad 5 libraries and on SnapEDA.
There was discussion of a Particle releasing Eagle part (which could be converted to KiCad) in the Particle Mesh Schematics thread.