[NanoEntity]

yes its possible, just point
forum.domain1.com to forum.domain.com/forumdisplay.php?s=&forumid=forumid

No need to edit global.php to assign urls.

Most Domains CP support point to point redirection services for subdomains.

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Stealth and Standard Website Redirection
With Website Redirection/Forwarding, we can send anyone trying to go to your URL (http://www.your-domain-name.com) to your "real" website, wherever it may be on the web. Simply tell us what your URL address is and we will forward any incoming requests for your domain directly to your site. This enables you to have a simple and easy to remember Web address. It also allows you to move from one ISP provider to another. Save money and get the identity you want!

IP Pointing
IP Pointing enables you to point your domain name to a specific IP (Internet Protocol) address on the Internet. An IP Address is a unique string of numbers, usually shown in groups separated by periods (e.g., 123.123.123.123), that identifies a computer connected to the Internet.

CNames
Canonical Name records (CNAMEs) act as aliases for the hostnames they are attached to. CNAMEs take on the web setting and MX record of their assigned hostname.