Upvote:1
http://matrix.itasoftware.com/ can do this. Hit advanced controls and type in USH. In October the lowest fare for AEP->USH->FTE was about 135 Euro. You can also specify a minimum connection time to make sure you have enough time in USH.
You can't book directly on ITA but you can go through 3rd party sites or directly through the airline using the info from ITA