Programs like the Excel to Arff Converter should be quite capable in converting of data from Excel format (.xls, .xlsx) to the ARFF format (.arff) so the data results can be analyzed with the WEKA Data Mining Utility, the original software for ARFF format.
Convert Microsoft Excel Open XML workbook and spreadsheet to Weka attribute-relation data.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert xlsx to arff on Windows
A XLS to ARFF converter
Convert from xlsx file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.