How can I change the FeedItem's sender?

In the case of the image, the user Alanna Clark updated an account and my trigger runned and posted the FeedItem.

But I want to change the sender ("Alanna Clark") for another user called "INTERFACE" when the trigger runs. What do I have to do?

//Feed PostFeedItem post = new FeedItem();post.Body = 'Msg body';post.ParentId = acc.Id ;insert post;