Can I ask questions that are not on the interview question list?

Yes! In fact, you should always ask appropriate follow up questions that drive to specifics so you can better understand your candidate's fit against the Job Cipher. 

For instance: if the candidate consistently says "We did this..." or "We accomplished that..." be sure to ask questions like "Great! and what was your role on that team?" so you can determine exactly what their role was in that success.

OR - if you're hearing information that conflicts with what you know from their resume, past interviews, or information collected previously, be sure you ask for clarification to unwind any information that would leave you guessing. 

However, be sure that your "off-script" questions have a clear purpose, versus asking questions that merely satisfy curiosity, or worse, introduce bias in your decision making process.