[{_id:12,name:"access",mobileNo:9153438340},{_id:13,name:"Apple",mobileNo:9153438343},{_id:14,name:"Dell",mobileNo:9153438344}]

db.collection.aggregate([{$project:{_id:1,name:1,mobileNo:1/** Include other fields from the existing documents as needed*/}},{$group:{_id:null,combinedResults:{$push:"$$ROOT"}}},{$set:{combinedResults:{$concatArrays:["$combinedResults",[{_id:15,name:"test",mobileNo:9553438343/** Add other fields to the new object as needed*/}]]}}},{$unwind:"$combinedResults"/** Unwind the array to get separate documents*/},{$replaceWith:"$combinedResults"}])