If I understand right your question, you are after the Discover Feature in the Extensions Manager.
First, make sure you have copied all the required files of an extension to their proper directories -wherever those may exist: administrator site, media, languages and so on-, otherwise you will encounter issues using this extension later on.
Then, go to the Extensions -> Discover (from left menu) - click the Discover button. It will list all extensions that may exist in the Joomla folder structure that have not been installed. You can select the one that you want to install and install it from there.
Well, you could upload the extension's files to the corresponding folders at the Joomla site you want to install it and then do the discover procedure.
Otherwise, you could re-compile the extension into a "installation bundle" in zip format and install it as usual. You can read its xml file to see its original files/folders structure, in order to organize the files accordingly, prior to compressing them.