I guess you think of xslt-filters, as xls is the old file-type of Excel, usually readable directly.
I’ve not attempted this myself and used another option to create xml-output but checked some docs that time, so “it can be done”. I remember a guy who converted data from a fitness-tracker via xslt-filter, but have to check, if I can find this. At the moment I only have an german article in linux-magszine and there are help-pages in LibreOffice…
The easiest way to combine files is to use csv-files (they have their own pitfalls, so be careful, concerning types). You can just append all csv-files by simple command-line appending cp *.csv >>out/dest.csv but you may need to remove Header lines before or after this and check, if the last line has \n etc.