yes it can be done, but opening the case is hard unless you use something like a dremel. The antenna connects to the card using a u.fl connector (which is used inside most laptops etc) so a u.fl to rp-sma adapter (
like this) would allow you to use almost any antenna.
That can work, but as the stock antenna is a wire taped to a piece of drinking straw, it's probably wise to upgrade!