Problems moving Time Machine drive from USB to Airport Extreme

Greetings compadres.

I'm using a WD external Mac hard drive for my Time Machine backups. I was using a USB hub and directly connecting my MacBook to the hub, thus a seamless connection to the external hard drive.

Now, I want to connect my external drive to my Airport Extreme for wireless back ups. I tried this before and I couldn't figure out a way for the system to recognize the drive to continue my previous back ups. Eventually, I just reformated the drive and started my back ups over.

Is there a way to switch to wireless and continue my old back ups?

Much obliged,

- robert