Any digital phone can be used on the AT&T network. If the phone has an original service provider other than AT&T it will have to be unlocked prior to being used with their service. To unlock any phone you have to call the company that originally provided service for it.
At one point in time, AT&T did support the Nokia n95, but since cell phones are always being upgraded and the services have changed over the years, the phone may be too old to work properly if it is activated.
AT&T does support the Nokia n95 phone. I've heard some saying that they had trouble getting the 3G network on it, but AT&T itself does support it and probably has a helpline available to help users set it up.