This happens automatically - but you can't use an event token unless you're sending an email that is tied directly to the event. E.g. you can't use CiviMail, because CiviMail doesn't know which event you're referencing. However, Scheduled Reminders and CiviRules can use event tokens.
Because they're not available in CiviMail, they don't appear in the Tokens drop-down menu normally. However, they appear if you create a Scheduled Reminder. The format of the token is
X is the custom field ID.
If you go to Administer menu » Communications » Schedule Reminders, the Tokens menu will have your custom fields listed. You can freely copy the token into other templates that you intend to use with Scheduled Reminders, CiviRules, as well as the "PDF Letters for Participants" action, and possibly some other Participant actions (e.g. Name Badges).