Just to add...
Tested some more and have found a possible workaround:
If you choose Administrator from the Assign Author dropdown, you can specify the real author's name from the original article in the Default Author field.
You have to set Save Author to Yes and Missing Author to Use Default Author.
This way, the imported articles from the feed are displaying the correct Author name.
I know it's not done automatically the way it should but I think it's an descent alternate way to achieve what you want.
The side effect of this method is that you have to make sure that each feed item you create within Ninja RSS to Content, is from the same author.
In other words, you have to create a feed of the original articles (on the site were the article feed is imported from) from a single author. As far as I could find out, you can't mix different authors within the same feed.
(And I doubt if mixing authors in one feed is even working in the way it should. But this is just a guess).
Again, in the meantime I will ask if Daniel has the time to take a look at this issue.
I hope this helps.