Hi there - typically it is better not to sum in the load script, as QlikView can make light works of aggregating many many rows at display time. If you do wish to aggregate in the load it will be something like this:
sum(Value1) as Value1,
sum(Value2) as Value2,
sum(Value3) as Value3
(biff, embedded labels, header is 4 lines, table is Sheet1$)
GROUP BY Line, Action;
Note it is the GROUP BY at the end that is doing the grouping, and as you assume in your post the SUM that does the aggregation.
The syntax is slightly different with different data sources - if you have problems post back with details of where your data is coming from.