A nested TOC is not possible in a Sony UI using only the buttons. However, the same functionality is available within an LRF file for the Sony by using the cursor to navigate the TOCs.
Using one of Harry's Omnibus ebooks as an example, you have a main TOC that may list the titles of the books in the Omnibus. Clicking on one of these will bring you to another TOC for that specific book. Again you would use the cursor and select to go to one specific chapter. (I use this quite a bit when I'm reading Harry Sherlock Holmes Omnibus.)
Now if Harry has set the name of each book to be a title, then it will show up in the Sony UI under the Contents area with a number to the right of each book. Push the corresponding button and you will be taken to that part of the Omnibus. The second TOC (the one for the single book within the Omnibus) cannot be navigated with the buttons and can only be navigated via the cursor.
There is no ability that I have ever heard of that allows one file on the Sony to reference another file.