Data and content extractions from Documentum are not trivial, but they are not that difficult either. For one Armedia customer we used a Documentum Docbasic (.ebs) script to extract content and write it to a directory and to extract data to XML.
Our customer wanted a very flexible solution. We chose Docbasic because it is obviously simple to use in a Documentum environment. In this case, we identified the content file by the r_object_id and by other key data, with the file extension coming from the original file in the repository. You can organize your target file system where the content goes as you see fit. The EBS script also extracted the metadata and created a XML file with the data attributes and values, as well as the file system path of the content on the target location. The script was capable of creating a CSV as well as XML file for the data extraction. The script also creates a manifest of the content extracted.
This then describes a very flexible solution from a general point of view. Once you have the content on a file system and the data in such a form, then you can import it anywhere. The business application would guide you to what data you need to extract, and how to map it to Alfresco.
