how to set the author of the feed item to the author of the feed source in feed importer?
I had a hard time getting this to work.

My current settings work.

Feed Importer
-attached to Feed (content type)
-RSS parser
-Mapping: Description to Description, Title to Title, Item GUID to GUID (Unique), Item URL to URL (Unique), Published Date to Published Date, and Feed Node: User ID to User ID.  It is important to have a unique identifier or you get duplicate imports.

The Feed Itself
-Set "Authored By" to a Drupal user on your site.   This lets you custom the user. Create a new user if necessary. I created a couple dummy users just to get their names to appear as feed item authors.