itunes syncing works by creating a copy of what you have selected to sync in itunes on the ipad/iphone/ipod. If you already have music on the ipad, but its not selected to sync in itunes on your PC/Mac then it will be deleted on the ipad next time you sync, to match what itunes has. However if your music is already selected in itunes, it doesn't attempt to copy it again, it will just leave it on the device. If you have your sync settings setup correctly, adding a single album should just be as easy as selecting it along with all your current music. If your settings aren't set correctly then you need to sort that out.
Alternatively you can manage music/videos manually, but I've never used that option, but I believe if you have this set up, you should be able to just drag and drop your new album onto your ipad in itunes.