You'd need a Joomla event specific to that registration process that you could hook into or something in the user object that distinguishes them from regular registrations vs event registrations so you could use CBs normal user registration trigger. You can then just use CB Auto Actions and its CB Paid Subscriptions action to give them a subscription.

I don't see why you'd need to CRON schedule this. You should be able to just directly act on the users creation process. Does your extension have any triggers for when a user registers? If so that'd be where you'd need to look. I'm not the developer of that extension so I can't really tell you the best trigger to be using here. If they're using CBs API to register a user then you should be able to use CBs triggers as well assuming you've a means of distinguishing their registration from regular registrations (even something as simple as a URL param conditioned against using [get_PARAM]).

That will trigger for normal CB registrations as well so be sure you're conditioning it so it's specific to those event registrations. How you do that will depend a lot on if a specific CB field is being set for those registrations or if there's part of the URL you can extract to condition against.