If you afford to spend for private socks5 then you can simply go to setting, use a proxy/socks and insert IP and PORT, relogin to YIM and you are connected via the socks5 IP
For a socks5 source I recommend 5socks.net or vip72.org
If you want free solution, then you can use TOR
Download and install vidalia bundle from torproject.org
Then you go on YIM settings -> use a proxy/socks server, and there you insert for IP 127.0.0.1 and port 9050. You chose protocol socks5.
There ya go, you are connected to YIM on random IP's from TOR network.
Hope that helps.