There doesn't seem to exist any direct way how .iwa files (iWorks document archives) could be converted to anything else. These files are found inside compressed documents from iWorks (.pages, .numbers etc.) and replaced the previously used XMLs that were utilized for the document structure. If you want to convert your document, you will need to convert it as whole, not just a part from inside.
It might actually be possible to somehow decode the .iwa files (they feature Snappy compression) and rebuild some kind of "old" version of iWorks documents, but that seems like highly unreliably and complicated way.Open IWA file Open XLS file
A preadsheet editing application for iOS
A spreadsheet application from Apple