The reason why this can't be done (just like that) is because msn messenger uses the Windows crendentional system to save the default password. This can only be done with 1 password, thus only 1 password can be associated with a .NET Passport (and this is not a MSN Messenger limitation). So unless Microsoft changes this inside Windows itself, this is a no go.
And because this means not only modifying how MSN Messenger works (easy part), but also how Windows uses the whole crendentional system and related .NET Passport system (and thus also other non related 3rd party applications) it means a whole lot of work. In short: don't expect this to be added in MSN Messenger in the near futur...
The only thing you can hope for is if someone makes a program/addon to save MSN Messenger passwords in another place, and to patch MSN Messenger so it can also "read" from that other place as well, just as the
Password Reminder plugin from Wouter...
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