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There's no way to customize that in Sigil. You'd have to change it manually, outside of Sigil (the filename and its corresponding entry in the container file), and then properly zip everything back up.
You do have me curious, though, how changing the filename would affect the conversion to the Kindle format at all. Not that the name really matters much, but I've only really ever seen content.opf or package.opf being used, myself.
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