The WD protocol allows cookie manipulation:
https://www.w3.org/TR/webdriver1/#cookies

Make it possible to set cookies to be used later on in capturing:

• suite.setCookie(name, value, options)
• suite.setCookies([ [name, value, options], ... ] )

options = {
 path,
 domain,
 secureOnly,
 httpOnly,
 expiryTime
}