[{"_id":5,nextTaskId:3,tasks:{"1":"take out the trash","2":"pick up milk"}}]

db.collection.update({_id:5},[{"$addFields":{"tasks":{"$objectToArray":"$tasks"}}},{"$addFields":{"nextTaskId":{$add:["$nextTaskId",1]},tasks:{"$concatArrays":["$tasks",[{k:{$toString:"$nextTaskId"},v:"next task"}]]}}},{"$addFields":{"tasks":{"$arrayToObject":"$tasks"}}}])