The existing contentstaging documentation specifies that it is not possible to define a feed for a subtree of existing content:
q: can a feed be defined on a subtree of already existing content?
a: not yet. It is recommended to have no content for either source or target feed sources when creating the feed
However, synchronization is possible if both source and target servers share the same data (for example, if target servers use a DB copy of the source).
In this case, the feed initialization event may seem to fail, but it should not be needed as the the 'remote_id' for the main nodes is already the same.