Google is actively migrating old tours into the new Street View. Not all tours have been migrated yet.
After your tour has been migrated by Google, it will show up in your maps contributions and will be available for import into TourBuilder!
To check if your tour has been migrated by Google, visit the place page for the tour and take a look at the attribution at the top. If the attribution is a clickable link that takes you to your maps contributions when clicked then the tour has been migrated to the new Street View and is available for import.
As an example, the attribution in the below tour published by LCP Media is clickable. When the attribution is clicked, it takes you to the contributions tab for LCP Media. This tour has been migrated by Google and is available for import by the user that originally published the tour.
In the example below, the tour attribution is not clickable. This tour has not yet been migrated by Google and is not yet available for import into TourBuilder. Unfortunately, you will have to wait for Google to migrate these types of tours to import them into TourBuilder.