This issue has not been addressed. The question is how to import an Excel generated worksheet into QB to populate an Estimate or Invoice. An IIF is a Quickbooks format file - So importing a QB file into QB is not relevant, and frankly a non-answer response.

Excel is a very popular spreadsheet application. It is nearly negligent IMO that QB can't import .csv or Tab delimited data files. 

Retyping tabular data again into QB is tedious and does add value or 'delight' to your customers.

I have numerous workers generating Excel table data estimates and invoices - import these is a essential feature in QB.