You would define a custom function and use map.

def customFunction(row):

   return (row.name, row.age, row.city)

sample2 = sample.rdd.map(customFunction)

or

sample2 = sample.rdd.map(lambda x: (x.name, x.age, x.city))

The custom function would then be applied to every row of the dataframe. Note that sample2 will be a RDD, not a dataframe.

Map is needed if you are going to perform more complex computations. If you just need to add a derived column, you can use the withColumn, with returns a dataframe.

sample3 = sample.withColumn('age2', sample.age + 2)