i think xavier misunderstood a little, because he shared code from the preenfm2 and you asked about the preenfm3, right?
the files he linked to are the source files of the firmware, so you can change things there, compile your own version of the firmware and upload it to your device. i did this many times for the preenfm2 but i yet have to setup the build environment for preenfm3.
hope this clarifies things a little.