You can already do that. For example...
https://vborg.vbsupport.ru/external....2&forumids=108 Will show you the latest threads in the vB3.5 Code Modifications forum. Just change the 108 to the forum id you wish.
You can even add 2 or more forums. Like this.
Code:
https://vborg.vbsupport.ru/external.php?type=rss2&forumids=108&forumids=2
The above will show the annoucments forum and the vB3.5 Code Modifications forum. Just add &forumid=xx at the end... Hope this helps some..