You can kind of do this if you happen to have two full osu! installs and not just the scores.db files. If you copy all of the replay files from one of the Data/r folders to the other and then delete the scores.db file from the install with all of the replays, it will recreate the file using all of the replays when you start the game.
Otherwise, it's not possible to do this without using some external software, but unfortunately I'm not aware of anything that exists with this functionality.