Yes you can add these, in Firefox it's called Search Shortcuts and a few are there by default. But there are a few reason why I prefer DDG:
1. I don't have to add those things in my Browser
2. When I searched DDG for something don't find good stuff I can just go to the search bar on the DDG page and add any !bang to query a search engine I like (!y for Yahoo, !g for Google, …)
3. No matter which Browser I use on which device, the only thing I have to change is the default Search engine and it's set up for anything I want to search. This is especially nice for me since I use Firefox on my Mac, Brave and Safari on my iPhone and Brave and Chromium on my Windows device