You have use the Bilingual API to add comments to an SDLXliff file. You can find an overview of the bilingual api here:  

In essence you need to create an extension to the sdlxliff file type using FileTypeComponentBuilderExtension class. You can read more on how you can do that here. Also you can have a look at this plugin source code which is extending xliff file type.

I've also written an article about how you can access segment tags using the file type framework which uses the exact mechanism as the one you need to implement for adding comments. You can read my article here:  

The main objectives of the Integration API are to provide ability for the developers to create new views and view parts, extend the ribbon menu, create AutoSuggest providers and  ways to hook on main Studio views (projects, files, and editor). This API is more about extending the UI which was something very limited with the other API's

Hope this helps.

Your workaround is working at the moment but you are taking a big risk since the object you are loading using reflection it's not part of the public Studio API. Not being in the public API means that SDL might change the current behavior without informing anyone and your code will no longer work. I'm not saying that the public API is not going to change but it will be done in multiple steps to give developers time to adjust their code and also alternate approaches will be recommended. 

By directly operating on segments/paragraphs you might break internal Studio cursor/selection logic. This is the main reason why the objects you can access using Integration API are cloned objects and not the actual objects.

I do admit that the alternate solution from my initial reply adds more complexity to your plugin and also  require more coding but for the time being this your safest bet. There are plans to improve integration API by creating operations that can update properties or can make operations on segments/paragraphs. This objects will still not be directly exposed since we need to control how they are updated. This should be released in the first part of the year but this is not confirmed and it might be postponed.

PS: Your solution is not entirely correct and I will remove it as an accepted solution since it might mislead other developers. I will leave it as a suggest answered but I would like the developers to read the entire discussion and get all the details.

With the release of Studio 2014 CU 10 you can use integration api to add/remove comments. Have a look here for more details: