“In the studio we’re so often created on,” David Hallberg, the American Ballet Theater principal who dreamed up the workshop, said recently. “I really wanted to give the dancers a chance to have a more inclusive part in the creative process.” Mr. Hallberg first introduced the idea of a choreographic workshop in 2010, but after a few tries it didn’t stick and he was too busy with his international career (and a major injury) to help keep it going. Now, he’s determined to make the Incubator a yearly fixture at Ballet Theater.