It is possible to “force” a category when creating and listing post, using short-code parameters.

Shortcode example:

[frontier-post frontier_cat_id="34" frontier_list_cat_id="34"]

In this case the category is “Frontier Query” which has ID 34 (you can find the category ID two ways, hoover over the category name in the list, or check “Show ID in category list” in Frontier Post advanced settings.

More info on shortcodes: http://wpfrontier.com/frontier-post-shortcodes/