Yes, you can manually upload past training rides, individually or in bulk, and use them as benchmarks.  Check out this article for instructions.

All current season uploaded activities with sufficient data to produce an associated load value will impact your predicted FTP (since those loads have an impact on your physiological system). 

In addition, current season rides you mark as "FTP Checks" will alter the curve of your predicted FTP as these rides represent actual observations of your maximum effort.