Well, I guess it's possible but I only wanted to ask if it's possible to show on the forum wether the server is online or not by using YSI or Mysql?

How do you want to connect it? I mean, do you want to connect it to a UCP, or to the forum accounts?

You can, using vBulletin

Or if you can't afford vBulletin (or don't want to spend so much money for it), you can just make a function in your scripts that creates the username rows in the forum database, just be sure to secure everything (hash passwords, e.t.c)

You can show online users and server status by using THESE. You only need to know how to code using PHP and how to add the code to your forums.