You can do this:
- put the Flash Player on USB on some directory
- modify chrome/chromium command line to add something like in this Windows example (suppose your USB have this directory layout: root\chrome\chrome.exe ... root\PepperFlash\pepflashplayer.dll version 1.2.3.4)
chrome.exe (other arguments) --ppapi-flash-path=..\PepperFlash\pepflashplayer.dll --ppapi-flash-version=1.2.3.4
This is unsupported for distribution since AFAIK pepper flash is not redistributable.