1) If the labels have been printed out, they can be scanned into Word using OCR. If you then place them one above the other with a space in between each address block, you can use the formula described in the MS support article using Method 1.
http://support.microsoft.com/kb/212335
2) If the labels have been saved in Word, you can use the 'convert table to text' and make a new list of addresses (in one column) put the word ADDRESS above the first address then a space before your first address block. This is your header. Make sure there is only one space between each address block. Now you are ready to make a table. First do a search and replace for Enter/ Paragraph marks and replace them all with manual line breaks. Then do a search and replace for 2 manual line breaks ^l^l and replace these with one paragraph mark ^p. This will give you a Paragragh/Enter mark as a delimiter between each address record. Using 'Convert Text to Table' you will now have your addresses in a table. Now name and save this file, and you are ready to make new labels using mail merge from this table.
Hope this helps!
Chris