With a few assumptions* you can simply copy the content out of the source system (Interwoven in this case) using whatever export mechanisms it provides, and then copy it into Alfresco using FTP or CIFS. For really large sites (10s to 100s of thousands of files) I'd suggest batch loading the content via a series of discrete upload & promote-to-staging steps. The size of each batch is mostly defined in terms of the total number of files (the total file size of each batch, ignoring obvious outlyers, is basically irrelevant), and I generally use a rule of thumb of no more than about 10,000 files per batch (on 2.1.2 and beyond - for 2.1.1 and earlier I'd suggest no more than 5,000 files per batch).
If you're using TeamSite's DCT and TPL technologies, you'll likely want to migrate those in addition to the content, and that involves re-implementing them using the equivalent technologies in Alfresco (XML Schema and Freemarker/XSLT, respectively). That would need to be done before the content (DCRs) are brought over, and the DCRs themselves may require some transformation in order to be mapped from the legacy format (defined in the TeamSite DCT) to the new format (defined in the Alfresco XML Schema). The exact complexity / effort in this case is somewhat dependent on how faithfully you decide to recreate your DCTs as XML Schemas (see the second assumption listed below).
In terms of workflow, I think you'd want to start from scratch using jBPM. TeamSite's workflow functionality (as I understand it) is scripting based, which is a rather different approach to most CMSes (which usually use some kind of FSM style workflow engine eg. jBPM).
Cheers, Peter
* I have rarely (if ever!) dealt with a "pure" migration exercise - usually a content migration also includes at least one of the following activities:
platform upgrades
refinement / redesign of the content model
site rebranding
content inventory and cleanup
copy editing to bring the content into compliance with a new style guide
etc. etc.
All of these activities (particularly the last 3) can significantly complicate a content migration exercise, regardless of the source and destination technologies.
