Adding Line Item issue

There has to be an easy way to add a New line to an Expense Report?

All I want to do is when the Done button is pressed on a line item, do some calculations based on the data from that line and insert the results in the line below it.

I have tried some versions of this function below.  This code just replaces the Current line with the new calculations.

function xHalfGSTPST()
{
var taxCode = nlapiGetCurrentLineItemValue('expense','taxcode');
if(taxCode == "10390")
{
var GST = 0.07;
var PST = 0.025;
//GET THE CURRENT LINE INFORMATION
var SubTotal = nlapiGetCurrentLineItemValue('expense','amount');
var CurrentDate = nlapiGetCurrentLineItemValue('expense','expensedate')
var CurrentCategory = nlapiGetCurrentLineItemValue('expense','category');