I do not know of any way to do that, there is only Facebook sign in (though you don't have to log into FB site you can use Play Top Eleven - Free Online Football Manager Game or the app on a mobile device), and teams can be connected to a single mobile device (when started on that mobile device, I do not think you can transfer a device team to another device, which is why the FB sign in is a good thing, it makes your team available in more than one device, like when you replace your phone you don't then lose your team)

If you don't like spamming your FB friends with game posts you can change that in FB settings. Also you can change FB privacy setting to make your FB page invisible to the public.

You could try asking at TopEleven Support but at this time there is only the two ways to sign on, and though it is possible to connect a mobile team to a FB without a team, I do not think it goes the other direction.