It's not that easy. Glyphs are smoothed (or not) when they are created, you can't just turn it on and off. You would have to regenerate all the fon'ts textures from scratch when changing this attribute. So, my point is, it is not technically impossible but it can't be a simple Font::setSmooth(bool).
Additionally, glyphs in pixelated fonts are not supposed to be antialiased even with smoothing enabled. So I'd like to do some tests; can you provide a simple (yet complete) code that reproduces your problem, together with the font that you use?