The nicknames in the shared file(s) should be in a plain-text format, which looks something like this.
You'll have to get your file into this format. To see the exact format, create some nicknames in Eudora. Then quit Eudora and open a simple text editor program like NotePad. Open the nndbase.txt file in your Eudora directory from within NotePad, and look at it. The format you see there is the format pictured above, and it's the format you need your shared nicknames to be in.
When you need to make changes to this file, do so through your copy of Eudora (since only you have write-access). Or if you wish, you can just replace the nickname(s) files in that directory; if you do it this way, however, be sure to delete the associated .toc file; then make sure you are the first person to access the Nicknames through Eudora. This is necessary because when the file is first accessed, Eudora will need to create a .toc file, and that's only possible for you (since you have write-access).