America Online may face a hard sell with its AOL Enterprise Federation Partner program, a new effort unveiled Thursday that's designed to <A HREF="http://news.zdnet.com/2100-3513_22-5671394.html?tag=zdnn.alert">link IM users at work.</A> Converting free consumer products like AOL's Instant Messenger, or AIM, into paid services tailored to a business clientele can be harder than it looks. While AOL estimates 14 million of its AIM users trade messages with each other at work, it and other Web titans have found themselves on the sidelines when it came time to sell their products to professional IT managers.