I was having the same problem. The file copying solution is simple enough but there really should be a GUI way to create a new page type based on an existing page type.

I'll post some steps here to save people some reading.

Here is how to create a new custom full page type.

1. Find the full.php file in the default theme directory. If its not there, go to dashboard->themes and click on inspect in the plain yogurt theme. A create button will be there if full.php is missing. Click on that.
2. Copy the full.php to the theme directory you want to create a page type for. You can copy the file to the 'default' directory if you want to create a custom full page type in the plain yogurt theme.
3. Rename the file to whatever handle you want.
4. Go to PageTypes in Dashboard and click on Add Page Type.
5. Enter whatever name you want but in the handle type in the file you renamed without the php extension.
6. Your new page type will be using the full page type.

I recently learned the hard way that it is important to be logged in as the super user (initial admin account) to have full capacity to implement new page types. Just sayin' ...