I asked the darktable crew about this, and their response was: don’t.
The problem is that there is a possibility of data loss during the conversion process, with little to no practical gains for doing so. EXIF is one place of possible corruption as well.
http://redmine.darktable.org/issues/9453#note-1
http://redmine.darktable.org/issues/10434#note-4
I think the general feeling is to ask what possible gains there might be from adding this conversion step (and to weigh it against the possible cons).
We already have libraries to read the CR2 files fine (I think? I’m not a Canon shooter - someone correct me if I’m wrong). If something changes in the future for CR2 files, you are not going to lose the ability to open your existing files with the existing libraries.
With that being said, I’m not sure of the easiest way to install only a portion required to fire up digikam. Doesn’t that get handled by your package manager w/o having to install a large portion of KDE?