Hi @cbaccar,
At this time, the file types available for import to QuickBooks Online (QBO) are CSV, Google Sheets, and XLS/XLSX.
An option I can provide you is to look for a third-party app that can convert your IIF file to any of those accepted file types I stated above. You can also search for a third-party app that can directly import the IIF file to your QBO company.
For both options, feel free to check out this page for a complete list of third-party apps that you can sync with QBO: QuickBooks Apps Store.
Keep this article for reference: Common questions about importing data to QuickBooks Online.
Let me know if you have other questions about the import process in QBO. I'll be sure to take care of them for you.