Currently Being ModeratedFeb 19, 2012 3:12 PM (in response to Fabrizio Zambelli)
if you already have one iCloud account signed into in System Preferences>iCloud, you can add additional accounts but not there - you enter them in System Preferences>Mail, Contacts & Calendars. This will enable you to access email, but you can't sync Calendars and the rest of it with any account except the one you signed into first, the 'Primary Account'. If you want to use another account fully you would have to create a new user account on your Mac and sign in there.
Incidentally, if a Family Pack sub-account has been successfully migrated - as appears to be the case - it's an iCloud account like any other: the fact that it was once a sub-account isn't relevant, so don't let that confuse you.