The "Extensions" folder in the profile folder is for extension files that Opera places there when you install an extension via its crx file (like when you drag a downloaded crx file into Opera or click "Add to Opera" at chromewebstore.google.com and addons.opera.com). The folder is not for the user to manually place things there. Opera will remove anything you manually place there.
So, really, you should just be reinstalling the extensions from the extension stores.
However, if you have the extension's install folder for an extension that is no longer in the stores, you can load that extension manually. You put the extension's folder (you can rename it to the name of the extension if you want) somewhere (like on your desktop or in your home folder -- Do not put it in Opera's "Extensions" folder), goto the URL opera://extensions, turn on developer mode, click "Load unpacked" and point it to the extension's folder (wherever you placed) it. Specifically, you need to point Opera to the folder where the extension's manifest.json file is at.
Before doing "load unpack", you should delete the "_metadata" folder in the extension's folder if it has one. And, you really should edit manifest.json with a text editor and remove both the "update_url":"value", and "key":"value", lines, especially if the extension is is blacklisted by Opera.
