The easiest way is to add a HDD is to use an external USB drive.
From my experience, you will have to overcome 2 problems:
1) Be able to auto-mount the drive at boot-up
2) In the event that server has to be re-started / re-booted will it be detected on boot-up
I say this because I have a usb dvd, in the event of a re-boot or restart, the drive is not always detected.
I know you do not want to hear this, but I personally would add or upgrade the internal HDD(s). My server runs 6 internal hdds using RAID 1 arrays ( in the event of a crash ) with 6 spares sitting on the shelf.
If you are planning on handling a lot of sites, then I would recommend a system like mine. Your customers / users will not be happy with any down time.