How to convert a spark dataframe into a databrick koalas dataframe?
To go straight from a pyspark dataframe (I am assuming that is what you are working with) to a koalas dataframe you can use:
koalas_df = ks.DataFrame(your_pyspark_df)
Here I've imported koalas as ks.
I know that you can convert a spark dataframe df into a pandas dataframe with 

df.toPandas()

However, this is taking very long, so I found out about a koala package in databricks that could enable me to use the data as a pandas dataframe (for instance, being able to use scikit learn) without having a pandas dataframe. I already have the spark dataframe, but I cannot find a way to make it into a Koalas one.