Hello Veronica,
Great question! You can create a Custom Contact Field at the Admin level of your account and you can import the "answers" to the contact field for contacts whose employee #s you already know. This field can pre-populate in registration if you've added the custom contact field widget into the registration process.
Here is more information on creating custom contact fields - Using Custom Contact Fields. I'm also including an article on importing contacts, if you need to reference it for the modification import where you'll be able to import the employee #s for the newly created custom contact field - Importing Event Contacts.
If you need any guidance during the custom contact field creation and/or contact modification import, the Customer Care team will be happy to walk you through the process(es)!
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Best,
Raana Willis
Senior Customer Success Advisor
Cvent
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