Hi Samuel,
Thanks for reaching out!
There is not currently an out of the box way to do this, but I think the simplest way to do it would be to use a custom registration form and then redirect to the import page (your-site.com/wp-admin/edit.php?post_type=tribe_events&page=aggregator), where they can add their url.
If you want to do something like have them enter the url as a custom field when they create their account and then automatically add the url into Event Aggregator, that would involve quite a bit of customization. We are not currently able to do customizations, but we do have a themer’s guide with some great info on customizing our plugins. We are also here to answer questions and help point you in the right direction!
I hope that helps – please let me know if you have any questions on this.
Thanks,
Jennifer