Here is an example to add an extension with Chrome with a remote server:

from selenium import webdriver

options = webdriver.ChromeOptions()
options.add_extension(r'C:\Downloads\Adblock-Plus_v1.11.crx')

capabilities = options.to_capabilities()
capabilities.update({'browser_version': '50.0', 'os': 'Windows', 'os_version': '10', 'resolution': '1366x768'})

driver = webdriver.Remote("http://127.0.0.1:4444/wd/hub", desired_capabilities=capabilities)
driver.get('http://stackoverflow.com/')

Note that it is preferable to set the Log level to WARNING…