I did it. I now share Mail\News and Bookmarks between Netscape 7.00 and Mozilla 1.2.1. It wasn't without a bit of trouble and work.
I first worked on changing the Mail\News to be in common since this seemed to be the easiest to do (ha!). [I may have mis-interpreted how many changes need to be made.] I decided to change all pointers in Mozilla Mail\News preferences to match that of Netscape 7 (did I have to do all that?). I had trouble here, and decided that the account names in Mozilla should match those in Netscape. I also, eventually, decided that I should close and restart Mozilla after I overhauled each Mail or News account. I got the Mail to work, but Mozilla froze solid while I worked on the News accounts. Mozilla thereafter froze every time I tried to open Mail/News, and this made further editing of preferences impossible.
I deleted the troubled Mozilla profile, choosing the option to delete all data files. It deleted the profile's directory, but, thankfully, did not delete any files in my default Netscape profile (which was backed up, just in case).
I proceeded to build a new Mozilla profile from scratch. This time, I created each new News or Mail account by copying and pasting items found in Netscape's preferences (you could get confused doing all this copying and pasting; I did.). I did this for all names and paths found in the preferences for each Mail and News account. After editing each account, I would first close Netscape and then close and re-start Mozilla. Voila! It would appear that all the desired newsgroups and mail are shared between the two browsers. (There should be an easier way.)
The bookmarks sharing was easier after I really realize that all "\" in the path had to be replaced with "\\" or things would not work. The line in my user.js for my Mozilla profile now looks something like the line below (really one, big, long line):
user_pref("browser.bookmarks.file", "C:\\WINDOWS\\Application Data\\Mozilla\\Profiles\\Nutcase's Account\\9gxilhof.slt\\bookmarks.html");
This points the Mozilla profile to my Netscape's bookmarks file.
I'll let you know if things get gunked up later since I just made the change an hour or two ago. BTW, depending on what you do with Netscape, it might be a good idea NOT to have the mozilla registry.dat Read Only.
Remember, I did this on only one machine so far. My procedure may not be the best. You need to be cautious when making these kind of changes, although the bookmarks thing was pretty easy.
UserAgent: Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.2.1) Gecko/20021130