Here is the guide how to transport MSN contacts into Jabber.
http://delx.net.au/projects/pymsnt/docs/user.html
You can't do it with the Jabber's client on the N900 .Then you must download a Jabber client ( I use Psi) for your computer .After transport all contacts MSN, you can log in Jabber'client on the N900 and voilà
It works for other IM like AIM, ICQ, Yahoo, .... All you need that you must find a Jabber's server that support IM transport:
List Jabber's server :
http://www.jabberes.org/servers/

Don't forget to dellte all MSN contacts you dont need in MSN Messenger if you you dont want to see its on your N900
If you have others questions , ask me ^^