How to convert a spark dataframe into a databrick koalas dataframe?
Well. First of all, you have to understand the reason why toPandas() takes so long :  

Spark dataframe are distributed in different nodes and when you run toPandas()
It will pull the distributed dataframe back to the driver node (that's the reason it takes long time)
you are then able to use pandas, or Scikit-learn in the single(Driver) node for faster analysis and modeling, because it's like your modeling on your own PC
Koalas is the pandas API in spark and when you convert it to koalas dataframe : It's still distributed, so it will not shuffle data between different nodes, so you can use pandas' similar syntax for distributed dataframe transformation
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.