I’m not sure what you are exactly referring to here as use post as post-parent. The post-parent post hierarchy option is available for all post types (except pages as they are special due to the root page concept) within Breadcrumb NavXT. When setting the post hierarchy to post parent, it will follow whatever the post-parent property is for the post.
It is outside the scope of Breadcrumb NavXT to establish the actual post-parent relationship (Breadcrumb NavXT displays the content organization; it doesn’t establish the content organization, WordPress does that). Adjacent/helper plugins such as Post-Parents can provide a GUI to establish that relationship, however they may be limited in functionality.
Lastly, if you need posts to be the parent of other posts, consider using an hierarchical custom post type instead of the built in Post post type. WordPress will provide the post parent GUI elements for you for hierarchical post types.