Originally Posted by
Stripe
To answer you original question, use matrix.itasoftware.com to build and price your itinerary, then pull up the "Book with Matrix" site. It will load your itinerary from Matrix and then redirect you to aa.com with that booking loaded. Unless it violates some fare rule this has always worked for me when aa.com's multi-city search won't give me what I want.
Matrix is best at forcing it, it will likely charge out as multiple one-ways though as the routing likely violates maximum distance rules (hence the error), which (likely) will be more expensive.