Creating a password protected excel is not possible with the standard exporter module.
As the module is using apache poi to create the files, reading this thread on stack overflow might help in finding a solution:
This will not really help you to protect your data from reading, it only protects data being changed.
Have a look at this site to understand the options that poi offers for encryption:
This will require some changes/ additions to the java code used by the excel exporter though.