The info below is based on my copy of the Mozilla Suite 1.7.12 which should be close enough to Netscape 7.2. (Others will post to correct me if this is not true.)
Open up the Mail component of NS 7.2. Click, in the toolbar at the top, on Edit|Mail & Newsgroup Account Settings. In the left pane which lists all of your accounts, scroll down until you find (probably at the bottom) "Outgoing Server (SMTP)" and click on that.
Now at the right, you will see all the settings for the "Default" outgoing mail server. To add a new account, click on the "Advanced" button in this section. A new window will open that lists all the outgoing servers. You will find, among the buttons here, one labeled "Add." Clicking on this should be what you need.
BUT, once you create your new account, go to its "Server Settings" and click on the "Advanced" button there and choose the outgoing server whose settings match that account.
Post back with any other questions. I'll be gone for a while, but others can answer your questions.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.12) Gecko/20050915