Since OVI is Jabber-based IM protocol, and the very Jabber is able to comunicate with other IM protocols, it is possible to add MSN, ICQ etc. contacts to your OVI account, just like you can add the Yahoo! messenger contacts to your MSN account and vice versa.
The secret is that you must register with the "transports" at one or more public Jabber servers.
1. Download and install Jabber client "Psi" http://psi-im.org/ onto your PC http://psi-im.org/ or the "talkonaut" client onto your S60 device, and log in with your OVI account.
2. In the menu you find "service discovery" item, in the adress section type one of public Jabber servers name and click browse, here is a list of servers http://www.jabberes.org/servers/servers.html but not all of them work for public use, so you will have to serach for the working one. For the beginning I will specify msn.jabber.org.au for MSN, and icq.fsinf.at for ICQ.
3. When you're done with that, in the tree below you find the name of transport and double click on it. Enter your MSN/ICQ username and password and that's it. You will recive invitation from the Jabber server and accept it as a buddy.
Now, all your MSN/ICQ contacts should apear in the PSI client in the form email@example.com for MSN contacts and ICQ firstname.lastname@example.org for ICQ .
You can rename them into whatever you want, and you are free to uninstall the PSI client.
When you log in your OVI contacts aplication on the phone you will see all your MSN/ICQ buddies inside.
I still can't find the working Google talk transport, so if anyone finds it, I will appreciate it.
It would be easier if Nokia installed these transports on it's own servers to make whole thing easier, so we don't have to beg other people for transports.