Can I add links to websites on Moodle for students?

Answer

Yes and it is good practice to:

link to the home page of the site, as far as possible avoiding a deep link to a specific page or resource (deep links can be unstable and are prone to being relocated or removed)
ensure the link opens in a new browser window (this makes it clear to students that they are visiting an external website, and means that Moodle will still be available).