I had tried using a 0.5x macro with wide angle conversion lens fitted to a Canon EF-S 18-55mm f/3.5-5.6 IS lens and during autofocusing, you can clearly feel the DC micro motor straining under the weight of the set-up. I sincerely believe IMHO fitting one to a Canon EF 50mm f/1.8 II Lens will stress its autofocusing DC micro motor even further.
Not to also mention the horrible degradation of image quality (additional induced perspective distortion, loss of sharpness, sever chromatic aberration etc. etc.) associated with the use of such wide angle conversion lens.