Quickbooks Online Pipeline - Add Invoice Line Item

I'm having difficulty figuring out how to add a line item to an existing QuickBooks Online Invoice using Pipelines.  If I choose the action "update invoice" and put in the info for the line item, it over-writes all of the existing line item data, leaving me with just the one line I was trying to add.

Is appending a line item to an existing invoice possible without have to resend all of the line items data again?  

Thanks in advance!
Mark