Is it possible to add a subform or some sort of report that would join invoice data from Zoho Books (via API or otherwise) into a Zoho Creator form.

 

Roughly:

 

The fields would be:

Account:

Invoice date:

Subform (or some alternative) of uninvoiced time associated with the account in the main form that included timesheet info and added an additional numerical value field (actual time you want to invoice). 

 

When the user clicks on submit the timesheet data would then update as invoiced/billed or uninvoiced/unbilled.