BeserksareOP, you can rename eras in the editor, but it is not possible to add new eras to the existing eras, You can group techs inside an era to create 'sub-eras' as it is done in edit: Meteorpunch´s Aeon mod and as it was suggested by Steph long ago.
...and you have posted your question in the wrong forum, This forum is for tutorials, references and guides, where you can explain others some deeper functions of Civ 3 or ask questions concerning those explanations - not to only ask new questions ( this should be done in the normal Civ 3 Creation and Customization Forum).