Do you mean something like:  'seo-forumz.com' ?

I'm not too sure whether spelling has any influence on SEO, but it might mean less SE traffic as I would guess that most searchers would not search on that variant of a misspelling?

Having a 'z' or and 'a' etc will not have any special effect in search engines. But if you have the best posts and good info shared in your forums, then even a worst domain will get a good number of search engine traffiic.