Hi Markus,
just quick hint regarding XLSX format - I think with CL_XLSX_DOCUMENT you are on the right path.
However, if the XLSX file you need to analyze is not very complicated, you can do it yourself with simple string or XML methods.
The XLSX format is just a zipped collection of XML files. Just take any XLSX file, change extension to .ZIP and look into the file.
In /xl/worksheets/* you will find xml files with worksheets - they contain cells and either value itself (formulas and number) or reference to list of shared values (strings).
The shared values are usually in /xl/* (sharedStrings.xml, for example).
Regards,
Dawood.