Hey and welcome to Meta @R4356th 
No, making changes to Discourse files on your server (in this context) means that you’ll either be unable to update your site or that your changes will be wiped with the next update.
The way to approach this is to either create a plugin or a theme / theme component.
Those are the methods your can use to extend Discourse to match your needs.
For your particular feature, you’ll need a plugin. I recommend taking a look at the source code of the bbcode plugin for inspiration.
https://github.com/discourse/discourse-bbcode