You can certainly embed custom XML files in an XLSX.
If you don’t want to convert to XML format, you could convert your file to base64 encoded ascii, and put that in a simple XML file. This would probably entail almost no work at all for you.
You can put the custom XML file in the root directory of the XLSX, or you can create your own subdirectory, and put the custom XML file there. It probably would be a little neater to put it into your own subdirectory.
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