if you google:
"waypoint-table" waypoint route "route-point"
you can find examples of FPL files.
They do appear to be xml files in a pretty simple format, which could be put together with a text editor. That would be error-prone, so you probably want to use a program.
It would be easy for a flight-planning program to output that format. The actual programming is trivial. Since most don't, I suppose there might be some legal issue.
Garmin do not publish the fpl schema (data layout), which would tend to support that idea.