We currently don't have a out of the box integration with other document management/cloud storage providers.
However, Clarizen certainly support Link documents.
As this is the developers forum, I can think of a solution that answers to that:
- You create a custom action which uses 'Open Url' to open a Web Application you develop.
In the request string you can pass along the entity type and external id.
- The web application provides the GUI to interact with your document management provider. When you're done, you can upload a new (Link) document through the Clarizen API, attaching it to the desired object (with the entity id passed in the URL).
I hope this helps.