Is it possible to create an unauthenticated nuget feed in ProGet where user credentials do not have to be supplied to access the feed?
Yes, just grant the default "View Only" role to the Anonymous user. If you're using LDAP authentication, then grant that role to the "Everyone" group.
Also note that if you running ProGet through IIS, you must enable Anonymous Authentication or else every unauthenticated request will receive a 401 authentication challenge.