Hi DGC,
Do you mean you have completed the migration batch, but you have created a few new items in the on-premises Exchange server and you want to migrate these items to Office 365?
As you have stated, you can resume the migration batch in EAC (Exchange Admin Center), this will start the migration again, it will skip the items already been migrated and only migrate the new items to Office 365.
For more details, please refer to: Migrate All Mailboxes to Exchange Online with a Cutover Migration.