I have seen the article on how to create a custom content type using Thunder in Visual Studio: http://docs.sitefinity.com/thunder-create-a-custom-content-type
However, the article does not tell how to set a Parent Type on the new content type, nor does the "Add New Item" wizard offer the option to do so.
(If I create a custom type using the ModuleBuilder, in the Admin web site, then I do get an option to specify the Parent Content type.)
Is it possible? Does it have to be coded by manually? If so, how?