Hello Chanty80,
There are two ways that you can go about adding these songs to iTunes. However, I would recommend removing the existing entries to avoid the appearance of duplicates in your library.
One way is to simply drag the desired folder onto the iTunes window while browsing your existing library. The other is via the File menu, by choosing the 'Add Folder to Library.'
Deleting files from the iTunes library and your computer
http://support.apple.com/kb/HT3842
Note that, as iTunes cannot locate the items that you'd be deleting, you don't need to worry about the songs in question actually being deleted. This process will only remove these songs from the list in iTunes.
Adding music and other content to iTunes
http://support.apple.com/kb/HT1473
Cheers,
Allen