Hello

I have some errors in my google webmaster tools and some forums with a poor quality (forum games, ... )

We can exclude forum to sitemap, but it's possible to exclude some forums with a noindex value ? In my case the forum games topics are not good for SEO, it's a poor quality content and google take some times to analyse these pages. 

I have some 403 errors with the do=add parameter. It's possible to add noindex for page with a add/edit tag ?

It's possible to personalize 404 error page, for deleted topics, with a "Content related" page ? (A search query with the topic title if rewrite is enable) 

Thank you

The only 403 / Access denied crawl errors I've noticed are forum threads which require a registered member/specific group to access the forum area. I am not sure if that is the best practice for handling crawlers though. I think Google wouldn't complain about these "URL errors" otherwise or? Probably the best practice would be to add restricted areas to the robots.txt

Yes you can add a robots= noindex, go to SEO optimization in your ACP and add the url of your forum

but this wont exclude topics from this forum from indexing

so you should change permissions of your forum and turn it invisible for Google