I know there are some ways to do cross login implementations using tools like OAuth 2, and maybe even auth0 (paid) has something that would make this job easier.
I don't know if the forum and the news portal are on the same or separate servers, but I think that connecting the user database can only cause additional complications in the future. It is always a practice to use different user/pass for each login, I don't see a big advantage of using the same combination on both domains.
I would rather go in the direction of automated conversion of a certain forum post into a news article on the site. Some kind of bot that the admin will call upon request and after revising the post. It would also make it easier for possible updates.