This should not be done on the same drive the Time Machine backup is on. First, the files will be deleted from the backup when that drive is out of space. Second, the Time Machine backup will eventually take up all the free space on the drive or partition holding it. Third, having the backup copy on the same drive as the original data won't help when that drive fails.
Purchase a second external drive, format it as Mac OS Extended (Journaled), and drag files to it manually.