You can ask visitor to become a fan before participating but cannot require them to share the campaign information on their timeline.
According to Facebook's guideline for fan page marketing, a sponsor cannot mandate participants to share specific information on the Facebook wall.
Facebook fan page guidline：https://www.facebook.com/page_guidelines.php#promotionsguidelines
How to encourage users to share your event?
Based on our experience and observation, visitors are more inclined to share events that they participated or events that give them a sense of achievement, such as poll, upload photos, check-in, comment & share, or discussion on a hot topic.