Due to limited support for .numbers files in 3rd party apps, it is likely that Excel workbook (.xls) files can be only opened and saved as Numbers spreadsheets (.numbers) only in the Numbers app for either Mac, iOS or iCloud. If you have access to Numbers, simply open .xls file in the Numbers and save it again as the default .numbers spreadsheet using the File ► Save as function.
Users without Mac can still generate .numbers file directly from iCloud, because it offers web based iWorks apps that are more than enough for a simple xls to numbers conversion.Open XLS file Open NUMBERS file
A preadsheet editing application for iOS
A spreadsheet application from Apple