How to append #rss to each RSS feed item's URL built with Views?
In the RSS view, look at the format section. If you're like me, you'll have the following

Format: RSS feed
Show: Content




Make sure to switch "Content" to "Fields", otherwise you won't ever see the "rewrite" options that the commentators keep mentioning above. Add all the necessary fields you'll need for the RSS feed.  You should get something like this (Ignore the field Hardcoded...)


Now, click on the "Content:Path" field (which you should've added), expand the "Rewrite Results" section, and tick the "Rewrite the output of this field" checkbox.
In the provided textbox, add your custom URL parameter. In my case, I wanted a hashtag, so I added the following:

[path]#rss

And, you're done! You may be asking, how did you know what to put in that textbox? Scroll down a bit and expand the "Replacement Patterns" section. You can add all types of stuff. I found this post useful. Of course, I discovered it after I had solved the problem. Still good to know.


Feels good solving my own problem, though special thanks to Remi who pointed me in the right direction. 

It seems pretty obvious after the fact and maybe that's why no one wanted to actually write out an answer but for beginners like me, it's obscure. Anyway, I wrote this out in detail in the hopes you won't get stuck here. Hope it helps :)