Peter,
AirNav do have an arrangement in place with a provider of FlightID / route information which is what Allocator was refering to so in that respect RB DOES autopopulate route information in terms of what they have access to when your connected to the internet.
However I believe what your asking is how you can import route information that you have access to in addition to the information RB comes with or AirNav can add. The data for routes is stored in the NavData.db3 database which can be found in the RB\Data folder. The database is in the SQLite format and the routes are stored in the table 'Routes'. The table columns are FN, NO, ND, NV, CH
FN - Flight ID (varchar max length 12)
NO - Origin (ICAO code, varchar max length 4)
ND - Destination (ICAO code, varchar max length 4)
NV - Via (ICAO code, varchar max length 4)
CH - Timestamp (when last checked and tells the RB software not to check the server for updates if less than 1 month old, format YYYYMMDDhhmmss, varchar max length 14)
I use SQLite Database Browser (free) to perform any batch updates I need to do.
Hope this helps.