How to add a point field in a bag file

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Currently, I have a bag file without the point field of timestamp. I changed the velodyne_driver and velodyne_pointcloud and added the field of timestamp when the msg of a packet is unpacked. But I don't know how to use it to add a point field into the existing bag file and how to check if it works. Do you think if it works that I run the driver package, open one terminal to play the existing bag file and open another one to record the data? How do I verify if the timestamp of a point is written into the file? Thanks.