How to have both dynamic and static sitemaps

Hi,

I have forums which are for archive purposes only so they are not updated, as well as forums that are active.

I want to have a static sitemap for the old forums and the dynamic sitemaps for the rest of ths site.

Is this possible and if so, how do I set it up? I can write the sitemap but would need to know what I should name it, and if I need to enable indexes or add it as an add URL.

thank you for your time.