I don't know about Mail, but Entourage can do it. Title your columns like First Name, Last Name, and Email Address. Export it as a tab-delimited file. Open Entourage, choose Import... from the file menu, and follow the steps from there. Basically the end result gives you new contacts in Entourage, which you can drag-drop onto your desktop, creating v-card files. Now open OS X's Address Book, create a new group (or select an existing one), and drag-drop all your v-cards into the group. This whole process should take about 4 minutes.