Each training session should address a specific topic.  Coaches have coaching points for the field players and should as well for the goalkeeper specific to the topic.  

Numbers down defending could address any number of gk topics such as communication, angle play, vertical positioning and through balls, etc.  To aid in the goalkeepers development it is important to focus on goalkeeper coaching points that will be highlighted during the training session.  Focus on a few of the points that are inter related that address a development opportunity with the keeper and build towards the goals set my goalkeeper and coaching staff.

Team Training for the Goalkeeper by John Murphy distributed by World Class Coaching shows how to integrate the goalkeeper into team training and the corresponding coaching points.

 PDF - Numbers down defending and Distribution - GK as an outlet for switching play.