Generally the answer to "is it possible" is usually "yes" with a possible additional condition that you might need to create a solution to do so yourself. So it's possible meaning that noone forbids you from doing it but not necessarily meaning that there is a built in way to do so using standard tools.
And this is the case with your question - by default there is no mechanism in Splunk that would encrypt the resulting csv/pdf/whatever - you'd need to take the existing sendemail.py and modify it so that it can do encryption - it would require some python skills and some time for development.
