That's two completely different things, one has nothing to do with the other.
An add-on wide-angle adapter will allow your existing lens to shoot wider, but will have no affect on making your DoF any shallower.
If you want to decrease your depth of field beyond what the camera will natively do, the way to do that is to use a 35mm DoF converter, such as the Letus you mentioned, or one of the higher-end units such as the P+S Technik.
I think the term "35mm" is throwing you off, because it's talking about two different things. With the wide-angle adapter, the 35mm is referring to the focal length of the lens, which is relatively wide. When talking about DoF converters, the 35mm is referring to the format size... as in the size of a 35mm film frame. Just by happenstance you've picked an wide-angle adapter that has the same number as the film format. It could just as easily have been a 28mm wide-angle adapter, or 18mm or such. I think that's where some of the confusion lies.
Remember if you go the DoF adapter route (Letus or whomever), you'll need 35mm format (not focal length) lenses to put on it.
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