How to force xml do not change undeclared namespaces in AS3?

I'm trying to load .xlsx file in to AIR program to search for specific strings there. Depending on the string, I append some data to the .xlsx file and export it.

The problem is, after the export, excel file is corrupt and I found out why. There are undeclared namespaces in the .xlsx format:

<t xml:space="preserve">Banner Set 1</t>

When the AS3 parses this line it becomes this:

<t aaa:space="preserve" xmlns:aaa="http://www.w3.org/XML/1998/namespace">Banner Set 1</t>

So, maybe you have some idea, how to make XML parser to ignore undeclared namespaces?