External Drive set up on Airport basestation, how do I use old backups?

[SPTN056]

I just bought and Airport Extreme Basestation from Bestbuy a few weeks ago. It works fine, and I just now got my 320GB Western Digital My Passport external drive hooked up and able to be used with Time Machine. I already had backups on that drive, and when I went to try to back up, it wanted to make new backups. I'm already using a little over 100GBs of space, so I A)don't want to have to back it all up again, and B)want the new backups connected with the old ones. Especially since I had to get rid of the very first backups when I got my Macbook in December. I don't know why it is doing this, but I need a way to get it to understand that the drive is what it used to backup to, and not a new drive, that way it doesn't have to start a whole new backup streak.