I'm not familiar with WxWidget nor WxSmith, but afaik SFML can only be integrated into WxWidget, thus you get a canvas widget you can render to, bit you can't place the widgets on top of SFML's rendering. If that's what you wamt, I don't see a reason why WxSmith shouldn't work for the surrounding widgets.
If however you want an 'in-game' (i.e. on top of the rendering, you'll have to use something else. See the FAQ on the GitHub wiki for an incomplete list of GUIs.