Database
[{"_id":{"div":"SP1"},"teams":[{"T":"La Coruna","P":3},{"T":"Malaga","P":3},{"draws":[{"T":"Barcelona","P":1},{"T":"Villarreal","P":1}]}]}]
Query
db.collection.aggregate([{$addFields:{teams:{$reduce:{input:"$teams",initialValue:[],in:{$cond:[{$eq:[{$type:"$$this.draws"},"array"]},{$concatArrays:["$$value","$$this.draws"]},{$concatArrays:["$$value",["$$this"]]}]}}}}}])