As far as I know, DataFrames are only backed by DataArrays. (I believe there's current work being done to upgrade the speed and type stability of DataFrames, in part by using Nullables.)
It might be helpful to write a little convenience function like f(n) = isnull(n) ? NA : get(n) I will say I've found DataArrays to be super finicky when inferring types. YMMV.
