To get the most recent historical Twitter tweets from a given Twitter user, do an import, and on the import screen:
Use a @(username) as the only term here. On the next screen, select the Only fetch once option:
This will go back in time for that user once, up to the maximum allowable, which is 3,200 tweets (as set by Twitter).
You can also use the limiting measures under the Additional Options on that screen to limit the number of items).
By placing these in their own archive, you can then directly create a bucket from that.