we've got to implement a document repository with Alfresco and is required that links to external documents (they could be an Url or a network uri like \SERVER\doc.pdf) should be available as regular alfresco-stored documents (tag, categorize and comment them).
I'm wondering whether it's possible to do that with Alfresco or we are going to develop that functionality ourselves.
You should be able to achieve this with Alfresco Bulk File System Import, which will allow you to load all the files from where they reside, into the Alfresco content store. In addition to that, you can use the Alfresco Content Stores to manage multiple content stores within Alfresco.