Like the title says, is it possible to add something like a Table of Contents to a
.cbz file, maybe as an additional text file used to better navigate the content? And if it is, how should it be structured?
Since the CBZ format is not standardised, there are some pitfalls if you try to achieve this.
The best you can do is render the ToC as an image and add it at the front of list of image files contained in the CBZ. Adding an extra file with the ToC information might break existing readers, if that ToC is not an image file.
An alternative is to load the data in the file comment of the CBZ file. This is limited in size (64K IIRC). There is an effort for metadata in CBZ this way (using JSON) that you could piggy-back on.
Disadvantage of a metadata solution is that there are no tools that currently display that information.
A bigger problem might be to get the information that you want to put into the ToC. Contrary to analysing the text of EPUB files after they are generated, the individual images of a CBZ have little to form the basis of a ToC.