Say I type \"sout\", the intellisense should expand it to \"System.out.println()\". Is there a way to adding such templates?
I've been Eclipse-free for over a year now, but I believe Eclipse calls these "Templates". Look in your settings for them. You invoke a template by typing its abbreviation and pressing the normal code completion hotkey (ctrl+space by default) or using the Tab key. The standard eclipse shortcut for System.out.println() is "sysout", so "sysout" would do what you want.
Here's another stackoverflow question that has some more details about it: How to use the "sysout" snippet in Eclipse with selected text?