Try this in the root folder .htaccess. It's tested and works on my site. I did this for another project in the past.
Code:
RewriteEngine on
RewriteRule ^forums/myfile.xml http://www.example.com/forums/external.php?type=rss [L]
The myfile.xml can be any name you want. The file does not have to actually exist. You can also modify the url to be selective on what forum ids are pulled and such and what type output you want. The example is rss. It can be xml or rss2 as well.