If you open Konqueror's configuration dialog and click on the fourth icon on the left[1], you should get a list with MIME types. Find application/x-rpm or enter "rpm" in the box above the list. Then you will be able to select an application used for opening RPM packets.
[1] I don't know what label it has in English version --- it's something about files, but I just can't find apropriate words to translate it.