Many parents often have problems with getting their children to sleep. A publishing company planned to produce a promotional video consisting of pictures from their published book and a new lullaby with a sleep-inducing effect. They requested that the new lullaby would be created through a collaboration of an automatic composition system and a musician. In our previous work, a melody generation method has been proposed to support the creative activities of a musician. However, this method requires too much intervention by a musician to meet the publisher's requirements. In this paper, we propose an automatic composition system that generates a new piece with a chord progression only by specifying some existing pieces. A case study is presented in which a professional musician completed a lullaby based on the piece generated by the proposed system.