How exactly would it work? Would this be automated, or are admins/users themselves submitting a custom URL for some of the users? Or would every profile be redirected to a common hard-coded URL?
Since Flarum is a single page app, having some profiles open in the SPA and some profiles open externally might be a bit of a challenge.
Also if you're replacing the profile page completely, you'll probably want to pull data/meta from Flarum to put on your custom page. Have you already implemented this?
If you want to add a link to an external page on the profile, you could use something like https://discuss.flarum.org/d/18775-friendsofflarum-social-profile or https://discuss.flarum.org/d/5791-masquerade-by-friendsofflarum-the-user-profile-builder
If you want to change the URL structure but not actually create a new page, this might help https://discuss.flarum.org/d/23872-custom-paths
If you need to add custom data to a user profile, you could also use the extension API to register a new tab under every user profile, just like discussions/posts/replies