By default, Feed to Post disables the Feed Items and the shortcodes from WP RSS Aggregator, since it overhauls the plugin by converting your items into full WordPress posts (or any other custom post type).
Legacy Feed Items
However, as of version 2.9.5, if you still wish to use regular Feed Items with Feed to Post, you can re-enable them from RSS Aggregator > Settings > Feed to Post, using the Legacy Feed Items option.
This will enable the Feed Items in the admin menu, will allow you to use the shortcodes, will allow you to use the Categories and Excerpts & Thumbnails add-ons, and it will also add Feed Item as a choice when selecting your importing post type (as shown in the screenshot below):
With these settings your items will import into regular Feed Items (when selecting “Feed item” as your “Post Type”) and you can display them on your site using the shortcode, just as you would when using WP RSS Aggregator without Feed to Post.
NOTE: When importing as “Feed items” the imported feed items can only be displayed using the SHORTCODE, and will not be imported as posts. It is NOT possible to import the same item as a “Feed item” and a “Post” within the same install.