How to union headers from multiple excel sheets and files?

Hi,

I have several excel files (sheets) with different headers.
How can I union all headers, change the headers of each sheet files, and padding the value with empty string if the sheet has no that headers before?
I need to append the data from all files, but for now pentaho works only when all sheets have the same headers.

For example, I have two files - file1.xls and file2.xls.
The headers of each file are:

file1.xls
file2.xls
How can I change the headers to header1...header6 for all sheets before appending the data?


Thanks for any help,
gintau