Different data types are being able to sync from Luminate into Raiser's Edge (Click here to see the list). Syncing additional gift a little different.
Additional gift or donation is considered a donation. In Luminate, normally additional gifts are for calendar events or e-commerce store purchases so the gift may be tied to an e-commerce transaction or a calendar event ticket transaction.
It will be synced as one transaction and not a separate gift. It'll be in the total amount of the original transaction whether if it was a purchase through an e-commerce store or an event ticket.