I'm sorry but I didn't follow you. The “Exclude Page URLs” option is designed to show pages, it doesn't hide pages. When you enable the "Turn WordPress to wpForo" option, it disables all pages on front-end. So you'll see all pages as 404 error. If you want to show some page on front-end you should put the page URL in the "Exclude Page URLs” option. So this will let your pages be shown on front-end.
Please read the documentation: https://wpforo.com/docs/root/getting-started/forum-page/turn-wordpress-to-wpforo/