To get you access to Live Messenger or other Live services, you don’t require a @hotmail.com or @live.com or similar account type. You can simply go to Passport, click the link under “Use an e-mail address you already have” and enter other e-mail address. This is very convenient because it allows you to keep using your current e-mail address in all Windows Live services, including Windows Live Messenger – except of course Windows Live Mail.
But if you have a Gmail account you are out of luck, as it seems, Microsoft doesn’t allow @gmail.com e-mail addresses to be used as a Windows Live ID.
Windows Live, Hotmail, Gmail, Google, Microsoft