I need more space on my hard drive and I have an unused 8gb USB stick.
How can I use that as an extra hard drive?
Short answer: You can't do what I think you're thinking, but it may be that what I think you think you're thinking isn't what you're thinking and so this will be helpful.
Long answer: You can't extend the partition from your hard drive onto the flash drive, but if you know that your flash drive always mounts to the same location (it will if you don't change it's name), you can put particular folders onto that space using symbolic links. Move the folder you want on the flash drive to the flash drive, then rename (or delete, but that should wait until after a successful test) the copy on your hard drive. Open a terminal and create a symbolic link from the place it was on your hard disk to the place it is on your flash drive. Note that it does not have to be in the same place on your flash drive as it was (for instance, on my Mac I do this with my Steam folder; I put it onto a flash drive so I can use it on multiple computers easily, so long as I keep the flash drive I don't have to re-download)
For instance, say you have a lot of content in your Movies folder: