@Vivian Kuo Broadcasting in Instagram is not recommended. You would only be able to broadcast to people who interacted in the last 24 hours, and Instagram is not made for Broadcasts.
My suggestion is that you use automations with Smart Delays to set up your logic and sent automated follow up a few hours after their initial interaction.
As for the Export Custom to System Field feature, works only to export information that you have on a Custom Field to a System Field (Email and Phone). So you need to have their email/phone on a custom field, and want to export it to the System Field.
If you have contacts with the Email System Field and Email Optin setup, then you can use Email broadcasts to your audience.
Hope this helps!