Yes, a user can still post time and expenses on a project that is marked as completed. All you have to do is to click on the Track Time option under the Time Activity section. Any information that will be added will still be recorded under the said project.
If you want to use it on your future transactions, you'll just need to change its status to in progress. Please see the screenshot attached for reference:
To give you more details on how the project works in QBO and on how to add time, please check out these links:
You can avoid posting additional time and expenses to your completed invoice by using the Single time activity or Weekly timesheet option. I'll walk you through the steps below:
Go to the + New icon.
Under Employees, choose either Single time activity or Weekly timesheet.
Enter the necessary information.
Click Save and close.
We also offer the following articles as your guide on entering hours on timesheets in QuickBooks Online (QBO):