Hey Boris,
You can setup a brand new channel in Marketo for webinars, and name the progression statuses whatever you want. If you are going to integrate it with GotoWebinar just ensure that you pick the 'Event with Webinar' type. Otherwise you won't be able to sync over the registration status from GoToWebinar (I learned the hard way).
Once you've done that, you just want to sync your progression statuses to the new ones you have created, you can do that in your flow.
Here is a screenshot of the custom channel that I setup.
If you need registered to be the first, you would just get rid of 'Invited', although I wouldn't recommend that as it is a useful progression status to use for segmentation.
Hope that helps!