I want to do a guerrilla usability testing and I was wondering if it is ok to ask to the participant to draw/sketch his thoughts as well?
Make the participant feel comfortable
Ask the user to do whatever they feel comfortable with to clarify an idea. Some people are going to freeze when you ask them to draw anything. Others will grab your pen and a napkin at the first opportunity to avoid fumbling with words.
Unless you are doing it with a very precise psychological goal in mind, you don't want to take a participant too far out of their comfort zone. They will clam up and stop thinking and communicating clearly. That won't do you or them any good.