A dm file can be easily made a true sis(x) file. Just open the .dm file that you got from the Ovi Store with a good text editor like Notepad++. In linux use ghex, it is available in Universe.
Launch Your Editor
Launch ghex and open your file. In the right hand pane, delete from the beginning of the file all of the below code, up to the z.
After that, save the file without the .dm extension. Bye bye!
photo credit: The Rocketeer