It's nice to see you here in the Community. I can share some information on how you can import your client's .QBX file created in QuickBooks Desktop (QBDT) to QuickBooks Online (QBO).
To do this, you need to convert the .QBX to a regular company file (.QBW) first. Here's how:
1. Open your client's file, then go to File menu at the top.
2. Pick Send Company File and Accountant's Copy.
3. Choose Convert Accountant’s Copy to Company File/QBW, then OK.
4. Select a location where you want to save it, then select Save.
5. Click OK.
That should do it! Please let me know how it goes or if I can be of additional assistance on the comment section below. I'd be glad to answer if you have any follow-up questions about the process. Take care and enjoy the rest of the day.