[{"name":"TEST1","department":"T1","skills":["JS","HTML"],"expY":5},{"name":"TEST2","department":"T2","skills":["Java","CSS"],"expY":3}]

db.collection.aggregate([{$match:{name:"TEST1"}},{$project:{name:"$name",department:"$department",expY:"$expY",skills:{$concatArrays:["$skills",["CSS"]]}}}])