Chrome extensions can be either packed or unpacked. Packed extensions are a single file with a .crx extension. Unpacked extensions are a directory containing the extension, including a manifest.json file.
To pack an unpacked extension, use the Pack button in chrome://extensions or use Chrome: "chrome.exe --pack-extension=C:\path\to\unpacked\extension --pack-extension-key=C:\myext.pem". See the extensions docs for other ways to do this that are more automation friendly. To unpack a packed extension, just unzip the file (you may need to rename the file from .crx to .zip for your zip utility to recognize it).
Installing extensions via ChromeDriver
Packed (.crx file)