I have a company that has as many as 20 different SalesForce orgs/instance, each with their own Chatter.  We are trying to do a merge into one singular org and would like to keep all past Chatter activity (posts, groups, files, follows, etc) from each of the other 20 old instances.  Is there a simple way to import (and merge) everything into a single org?  If there is no "easy" way to do this, is it possible to do it through the REST API (using a standard Java application)?

Notes/Constraints:

This is a one-time process - there is no need to check for duplicate messages

A user may exist on multiple orgs and the user on the master org should have all Chatter-related data from their other accounts

A user will be identified by their email address

All files hosted within each Chatter instance should be moved over to the master instance.

Data Export does not appear to be the way to go since it appears to match on internal IDs rather than the email address field we have to match on

The migration tool appears to be mainly for migrating data from Dev to Prod - if this is the correct tool for this task, I'll take another look at it.  It would have to be smart enough to match Chatter activity based on the email, though.