Possible to use airport extreme as second hard drive?
Do you mean a USB drive attached to the AirPort Extreme?

The router does not itself have a hard drive. And the facility to add a hard drive is meant as a backup or network storage not as a second hard drive per se.

That being said, you could use this option for storage of seldom used files that may be taking up too much room on your MBP. But accessing them will be much slower than files on your computer. If that is acceptable to you then it will work out well for you.

Good luck!