Conversion of Dataset into DataFrame
A Dataframe is a Dataset[Row]. any schema existing in the Dataset will be kept in the Dataframe.
Regarding the question on conversion, there is a simple way by using the .toDF() method :
val myDs : Dataset[Member] = someDs
val myDf : Dataframe = myDs.toDF()
Now there is a conversion in your method signature (Member => ExtMember) that seems to require some custom coding