How to insert an object which has random fields and fixed fields in MongoDB Java?

My libraries are:

implementation group: 'org.mongodb', name: 'mongodb-driver-sync', version: '4.3.2'
implementation group: 'org.mongodb', name: 'bson', version: '4.3.2'

I have an object like this:

	@BsonProperty(value = "file_names")
	public List<String> fileNames;
    @BsonProperty(value = "file_names_hash")
    public List<String> fileNamesHash;

I have to fetch data from multiple MongoDB collections and join them and add these two fields to them and insert them into another collection. How do I add these two fields to the List<org.bson.Document> which I will get from the join?