Would someone tell me if it is possible to save as Microsoft Excel (
.xls) in Calligra sheets?
Using calligra you can only open ms documents, but you cannot save them in .xls , you always will have to save them in .odf(Open document format). Sorry to be the bearer of bad news but that's one of the important functionalities that is missing in calligra.
You can use this url to know more about it : http://maketecheasier.com/is-calligra-a-great-alternative-to-libreoffice/2012/06/18
"One thing though, calligra doesn’t support saving to Ms-Word .doc and .docx format. It only supports the Open Document Format (ODF). You can open, view and edit .doc and .docx file, but you can only save to .odf format."
Unfortunately, Calligra does not (yet) have a filter to create/save to Microsoft formats. The only workarounds are to save in the Open Document Format (ODS for a spreadsheet) or as a Google doc.
Excel 2007 or later will open ODS files, but you may need to download a "reader" file to make that work. Otherwise, you would have to go through LibreOffice/OpenOffice to get an XLS or XLSX file for Excel to open.