[{registration:[{"_id":ObjectId("5e844235ead49b7ff33962d3"),"date":"2-jul","firstname":"John","lastname":"Doe","email":"john@doe.com","education":"University","gender":"male","age":"32","workshop":[2,3,6]},{"_id":ObjectId("5e844235ead49b7ff33962d3"),"date":"2-jul","firstname":"John","lastname":"Doe","email":"tina@hotmail.com","education":"University","gender":"male","age":"32","workshop":[2,3,6]}],"known":["tina@hotmail.com","sanders@hotmail.com","kyrana@hotmail.com"]}]

db.collection.aggregate([{$addFields:{registration:{$map:{input:"$registration",in:{$cond:[{$in:["$$this.email","$known"]},{$mergeObjects:[{emailIsKnown:true},"$$this"]},{$mergeObjects:[{emailIsKnown:false},"$$this"]}]}}}}}])