Via the GUI, no, unless your TV series volumes are listed as Unidentified in which case, you can identify them via the GUI. RTVHD grays out Series even though SeriesName is not a primary key in the database.
If your DVD rips are truly a TV series, then the TVDBID in the Series table (or RTVHD Settings' Series' tab) needs to match what TheTVDB.com has. Also, the following assumes that the DVD rips were identified correctly otherwise and have the correct MCID in the Series table, the Episodes table (for each episode) and the FavoritesList table (if added to one or more Favorites lists).
Via the "RecordedTVHDdb.s3db" SQLite database, the Series can be changed. The SeriesName would need to be edited in the Series table for the combined series, edited in the Episodes table for every episode of the combined series, and edited in the FavoritesList table if the combined series is on a Favorites list (for each Favorites list that it is on). Before attempting any changes, the Recorded TV HD Service service should be stopped and the database should be backed up.
If you're not comfortable doing SQL(ite) queries and updates... and if you're not familiar with the RecordedTVHDdb database schema, then I wouldn't advise doing this. You might be better off ensuring that your folder names and filenames match what TheTVDB.com has for the TV series and ensuring that you are using a filename format that matches one of the many that RTVHD supports. If TheTVDB.com's data is wrong, then you'd be better off working with the website directly to get the data fixed.