Good Evening, @AdamR123. 

Thanks for joining in on this thread. I've got you covered. Yes, it's possible to import sub sub-accounts and grandchildren accounts.

All you need to do is put another colon after your sub-account name. Here's a visual and written example: 

"Utilities: Gas: Truck Gas" 

If you still need the steps about importing your Chart of Accounts, here's the link that can provide the details: Move your Chart of Accounts to QuickBooks Online. 

For the future, here's a guide that can help you and your business manage default and special accounts in your QuickBooks Online (QBO) account. 

After adding the extra colon between the accounts, you should be able to import them successfully. If you have any other questions, feel free to ask. I'll always be around to help. Have a fantastic day!