This feature has been updated to allow for drag and drop and lookups using functions. More details can be found here.
Data Synchronisation Studio has a new feature to make it unbelievably easy to link data by way of Lookup Columns. A column can now link to any other data via a special Lookup column, you could do this with Dynamic Columns before but now it's just click...click...click.
You can connect SQL Data to SharePoint data for example or Join SQL Data across servers like SQL and Oracle or whatever you get the idea anything to anything.
To get started connect your data source as before and select the column that represents the Link or Join
Then click the Icon in the Data source Toolbar to Add a Lookup
Once you configure the connection details you need to confirm the Lookup Join details which is essentially the Data Type and Column.
Now once the Lookup loads you will see all the columns from the related Data source listed and you can add any of them to the Schema Map as if they were already in the source.
That's it really easy lookup columns in Data Synchronisation Studio, this feature is in release 3.0.444