Unfortunately, 3200 is the maximum number of tweets you can retrieve from the REST API for a given user.
To get more, you can use the Streaming API to track a set of users and get all their tweets, storing them for later.
To get tweets from the past - one of the historical GNIP APIs http://support.gnip.com/apis/, or asking the user to provide tweet IDs from their archive is the only way to do it.
