This will depend on which versions of Netscape you are using. If you are using the newest versions of Netscape 6.X with Mac OS 9 and OS X, everything will work out fine. You will have to create a series of folders in your Mac OS X Library folder. It will look something like this:
/Users/Your_User_Name/Library/Mozilla/Profiles/default/
In your Mac OS 9 Preferences folder, inside the System folder, There will be a folder for Netscape 6.X. I think it's labeled "Netscape Users" or something like that. Somewhere in there is a folder with some meaningless letters and numbers with a ".slt" extension:
[Letters_&_Numbers].slt might look something like pmc4jks7r.slt.
This is the folder that contains EVERYTHING you need. Copy this file (Option-drag) the [Letters_&_Numbers].slt folder to the default folder inside the foldrs you just created in your Library folder. The full path will be this:
/Users/Your_User_Name/Library/Mozilla/Profiles/default/[Letters_&_Numbers].slt
Startup Netscape X 6.X and it will work just like in Mac OS 9. The preference files, mail, etc... are identical between Netscape 6.X in Mac OS 9 and OS X, it's just that the preferences are stored in different places.