I am new to SS. I have utilised the existing blog module to manage the blogposts on my site. However, I want to have a dropdown box on the BlogHolder page (list of blogs) where you can select blogs by category. I do not want to touch blog module as it's maintained by someone else, so I want to add another controller action that can be called via Ajax when dropdown box is toggled. So far I have only found extensions that I can add to add more fields to data models, but in this case I don't need to change structure of any blog pagetype. How should I add more controls to existing modules?
Do you plan on using the already existing tag(s) field as categories or do you want to use a separate managed list of categories?
If its the latter you could try using a module I made: https://github.com/IOTI/silverstripe-blogcategories. It does not have a built it dropdown for selecting a category on the front end but you could build one yourself. If you don't want or have need of the module you could look at my code and see how i added extra controller actions to the blog holder i.e it uses
"yoursitelink.com/blog/category/category-name" for filtering blog posts by a selected category.