First, you can try the info in this link
, to see if it works for you.
If not, then do what I did to find that link, www.google.com/linux,
use the search keywords "dual boot" mac linux, and start reading.
PS: the subject line in your post doesn't seem to have anything even remotely related to 'expanding partition', as in re-sizing. Did I read incorrectly? Did I miss something?