If you need to export the data from .numbers spreadsheets to XML format and do not have the access to OS X and iWorks, your best option is to download the open source LibreOffice suite and use the Calc program to perform this conversion. Latest versions of LibreOffice added support for iWorks formats so this seems to be the easiest workaround.
Convert Apple Numbers spreadsheet to XML document.
We have found one software suitable in our database for this conversion.
Multiplatform software - convert numbers to xml on any platform
Full-featured spreadsheet tool from LibreOffice suite
Convert from numbers 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.