... create a link to an existing (external) page?

Ideally, you don't since the purpose of a Content Management System (CMS) such as Drupal is to keep all links self-managed. Instead, you should use cross-Drupal page reference as explained in our FAQ ... create a cross-reference to another page.

But in case you really (really) need to do this (pages are on a remote site and so on), here is how to do this:

• Do the usual -  add child page from a particular point
  For example, our example will Embedding
• A new page creation will appear. Chose a name for this page.
  In our example Old Requests
• In the field path-alias, enter the full URL of the target (remote) page.
  In our example, http://star.phy.uic.edu/twiki/bin/view/Main/EmbeddingStatus?cover=print
  Warnings
  1. Some CMS allows for a URL path to be un-terminated by "/" and make a difference between a URL like .../BLA and ../BLA/. Unfortunately, your Web CLIENT may not comply. Entering a non slash terminated path as reference will cause the Browser to add the slash hence the link fail.
     Solution: In our example, we circumvented this by using a ?cover=print syntax
  2. After you click submit, you will NOT easily be able to re-edit the Drupal page and change the alias unless you have the node reference number.
     Solution: In our example, we kept a link to the original Drupal node on Embedding and strongly suggest you do the same.
• Providing you read the previous warning, click submit.
  A menu entry is now added, referring to an external page and link.