Add SPListItem in a particular folder.

 private void AddSPListItemByRelativeUrl(SPList list, string relativeFolderurl)
        {
            //Add the files in sharepoint list under this folder.
            //If we want to add an item under "myFirstfolder/mynextfolder" folder, then the relativeFolderurl must be Lists/myList/myFirstfolder/mynextfolder
            SPListItem newFileItem = list.Items.Add("/" + relativeFolderurl, SPFileSystemObjectType.File, null);

            newFileItem["Title"] = "My new folder";

            newFileItem.Update();
        }