Hmm... an exact duplicate of
a thread
that started in the official Joomla boards over a year ago, the last reply two weeks ago...
...I am too unfamiliar with Sharepoint on a technical standpoint, however the solutions in that existing thread are quite concise and logical.
My personal thoughts are, if you run a corporate network that needs the specialization of Sharepoint over Drupal or Joomla (they are all different CMS's remember) and can afford the costs, then surely you can find the resources to code such a thing...?
What johntallon says in the first reply is accurate, I suppose it could be done with XML remote procedure calls in Joomla...
...this I know how to do but I know nothing of Sharepoint. If you find a Sharepoint+Joomla experienced coder i'd be happy to release a sample XML-RPC plugin for Joomla. Otherwise, best of luck - it's out of our field