You are absolutely right. Just open the zip files from the original DLC (latest version which is 2.0) with a program like winrar. Then drag and drop the dds files from the latest add on into it. Saving is not necessary as far as I know. But make sure that you put the right file into the right zip. Meaning If you open the "SummerOther" zip you have to drag and drop the content from the Summer other folder.
If you want to double check that you did everything right, you can delete the old noise_1.bmp files from the zip. If you see the new noise in game, everything is fine.
AM