In the grade book, Moodle does offer the ability for some grade items to be "extra credit." I am not exactly sure if this idea mirrors your "experience points," but I thought that I would mention it to you. I use bonus point in my courses, providing a point for whenever a student identifies an "error" to me. This helps sharpen students careful reading of information.
I also use "bonus activities." These are optional, intended for students who like to go beyond the normal expected learning. I put these into their own grade category in Moodle. Students can increase their overall grade by up to 2% by getting bonus points.