It is possible to import  Sub Total Lines  in Invoices / Estimates using Excel Transactions.

How to import Sub Total Lines ?

Kindly map one of your file column to "Show Sub Total" field and fill with values like "TRUE" , "T" , "YES" or "Y". 

The particular row which has "Show Sub Total"  should have the information like below image 

 It will be reflected in QuickBooks as below