Sump pump pit fill-in / Radon mitigation
No, it probably wouldn't be.  Radon can come in through many ways, not just via the sump pit.  It can come in through cracks in walls or the foundation, as well as other ways.  See http://www.dea.human.cornell.edu/sites/default/files/pdf/radon_illustration.pdf.  If you do fill in the sump pit, the radon will probably find another way into the basement.

If you wanted to try it and see what the impact is, I would suggest just buying a premade radon sump pit cover and installing it.  Make sure you caulk around the perimeter.  I would suggest against filling in the sump pit itself, were I the homeowner, unless I'd done something to mitigate against whatever water infiltration the sump pit and pump was put in there for in the first place.