This is now possible using the Advanced Drive service:
https://developers.google.com/apps-script/advanced/drive
When using Drive.Files.insert, simply set the optional parameter “convert” to “true”.
var file = {
title: 'Converted Spreadsheet'
};
file = Drive.Files.insert(file, xlsxBlob, {
convert: true
});
This was also obtained from the above given issue