Don't know what OS you are using but this can be done with W2K
Using Explorer open the folders under Profiles (your profiles are under Mozilla) and then the folder that contains your old mail (open the folder that's named with letters and numbers the Mail folder is under there, you will have two or more, open the one with your old mail files). Look to the right panel to find the mail files you want. You can drag or copy the mail file/files you want from there to your new Mail folder. Rename the file first, for instance if it's the Inbox rename it Inbox2, else it will overwrite your new Inbox. The file will appear as a new folder in your mail program.
OR you CAN use OE to import your Netscape mail. It's odd but N7 will not import N7 mail but OE will. Go to Import in Outlook Express and select Messages. You will be given a long list of email programs, select Netscape Communicator. It will either have the path to your mail folder in the slot or it will be empty. If it's empty or the path is wrong, using the Browse feature browse to your old Netscape mail folder. It will import all the email folders.