XMind brainstorming software supports export to all kind of formats, including Microsoft Excel spreadsheet (.xls, .xlsx). However, not all export options are available in the free version of the program and some might actually need a commercial license. Some viable alternative would be to find another brainstorming tool that supports Xmind formats and also allows export and saving to .xls files, but choices for xmind to xls conversion are limited.
Open XMIND file Open XLS file