Generate Random Strings (with millons of characters) in excel

I am looking for an add-in for Microsoft Excel 2013, that can generate a truly random string of any characters (millions of them), especially (A-Z) (a-z) (0-9). Please let me know what add-in to install or command I can use in Excel 2013 to do this task. The RAND and RANDBETWEEN functions return bigints, not numbers (or strings in this case), so they will probably not work. Thanks.