[{"key":1,"users":[{userId:1,scores:{failures:2,wins:4}},{userId:2,scores:{failures:0,wins:2}}]},{"key":2,"users":[{userId:2,scores:{failures:2,wins:4}}]}]

db.collection.update({},[{$set:{"users":{$reduce:{input:[2,3],initialValue:"$users",in:{$cond:[{$in:["$$this","$$value.userId"]},{$concatArrays:["$$value"]},{$concatArrays:["$$value",[{userId:"$$this",scores:{failures:0,wins:0}}]]}]}}}}}],{multi:true})