Database
[{_id:"something",topLevelProp:"top",obj:{prop1:{a:"a",b:"b"},prop2:{a:"x",b:"y"},prop3:{a:"x"}}}]
Query
db.collection.update({},[{"$addFields":{"obj":{"$objectToArray":"$obj"}}},{"$addFields":{"obj":{"$map":{"input":"$obj","as":"o","in":{k:"$$o.k",v:{"$mergeObjects":["$$o.v",{anotherProp:"$topLevelProp"}]}}}}}},{"$addFields":{"obj":{"$arrayToObject":"$obj"}}}],{multi:true})