Can I add time spent after a job has been invoiced?

No. Once a job has been invoiced, whether you used Create invoice or Mark invoiced, the job is locked. You cannot add or remove logged time, materials, or invoice line items.

You can still add or edit hours after marking a job complete, as long as it has not been invoiced yet. To unlock an invoiced job, use Remove invoiced status. Follow Editing time or line items after a job has been invoiced for the complete steps.