i'm not sure you can unless you have root, as the mount command requires this. I am rooted so i forget what the stock recovery is like, maybe there is a menu option to do it?
I may be wrong on this but I'm not sure you can on the stock recovery menu.
If you have SPRecovery on your phone you go to "mount options" and select it with your camera and the go to "mount /sdcard" and select it the same way.