I a need to add a small query string to some menu items on a website. For example, if Joomla gives me an article URL like Category/article/ and I need my viewer to land on category/article/?app=1. can I do that with rereplacer? when I make a menu item with joomla, it does not allow me to "edit" a URL to add query strings,
Yes, you should be able to do that with ReReplacer.
How to set that up depends a bit on what urls (format) you want to include. And if it should only be done in the menus or everywhere on your site where those arms are encountered.