Is it possible to sync timesheets which are not associated with any task?

Yes, these timesheets are classified as General Timesheets and they will be synced by default once you enable the integration.

If the General Timesheets are not imported from Zoho Projects, you can follow the steps below:

• Go to Settings at the top right corner of the page.
• Select Zoho Apps under Integrations.
• From Zoho Apps page, click Zoho Projects.
• Click Show Details to enable General Timesheets.