Version 1 has the option to publish the output of a flow to Tableau Server as a data source. I expect future versions will have much tighter integration with Server, though specifics haven't been announced yet.
Definitely download and give it a try - I'm loving it!
Have a wonderful day!
At the moment, it is not possible to use a published datasource on Server as an input for your Tableau Prep flow, but I expect this to be added in a future version at some point.