Stephany Chang - great question. There are some data mgt solutions that provide this as part of their overall solutions; so a bit on the heavy price and solutions side. There are some good tips/tricks to work with CSV files, Excel spreadsheets, macros, etc to help expedite the manual work efforts. In addition, we have a point solution on just this type of service and need for raw data files to clean and import into Markto. Feel free to DM here on the community and we can set up a call to talk to see if it will work for you.
why can't you streamline the process by digitising lead capture at the events and directly into Marketo? build your event program templates with a batch program that you set to run the day after the event is over which automatically sets anyone still with "Registered" status to "No Show"