Have you looked into the documentation for the Matlab command 'xlswrite' ?
You should be able to do something along the lines of:
xlswrite(filename, data, 'Sheet1')
But since the xlswrite function only accepts two-dimensional numeric, character array, or string array, or, if each cell contains a single element, a cell array, you'll have to reformat your data before putting it in.
If you'd like to add a new sheet just change the name of the sheet in the xlswrite parameters. If it the sheet doesn't exist it will be created.