Hi there.
I would use the offset formula for named ranges. It's a one off task and you should then be able to click Refresh All to update the pivot tables.
Let's say sheet 1 is named "Data". It has columns A-D, and 10 rows of data.
Create a named range called "Data", and add the following formula to the Refers To section:
=Offset(Data!$A$1:$D$1,0,0,Counta(Data!A:A))
This assumes that column A will have no blank values, as it needs to count each cell.
The pivot table for the "Data" tab will now need to have the data range updated to "Data". From that point on, any additional rows added to the data will be taken into account on the pivot table by simply refreshing.
This is my first response so let me know if that makes sense...