Hi!
Not as is. The post object only retrieves posts from the current wp_posts table in the database. Each blog in a Multisite get’s their own wp_posts table.
You could add this code yourself .. I think best practice (without too much hassle) would be to in the field-file check if this is a multisite. If so output another option where when setting upp the field you choose from which blog to fetch posts.
Thing is that this would not allow you to retrieve posts from multiple blogs in the same post-object field. The issue with retrieving a lot of posts from many different blogs is mainly performance. For each post the code would have to switch over to a different wp_posts table and do another query.
I think you should have a look at the post indexer plugin from wpmudev: https://premium.wpmudev.org/project/post-indexer/
