can you open the SMZIP files with winRAR? ^^ you could copy 1 of the smzip's wich you know works, open it with winRAR.. add the songs from the downloaded RAR archives, and delete the files that were in the SMZIP archive.
that way it shouldnt change the archive extension, but it would replace the songs in it. then save the modified archive under a differnt name, and put it back into the folder =]
Not sure if it will work.. but i gues its worth a shot. ( this is the way I soved many of my previous file extension problems =D )