Sure it is, using the command console. You just open it up and type "save" (or "savegame" forget which) followed by the name you want to give it.
You cannot actually "rename" a save file on PC . You can however use the console command to save a file with a specific name. The command is "save" and it will override saves with the same name without prompt.
Only drawback is since you cannot type quotes, your save names cannot contain white spaces.