Multiple idents are possible..
/server -m irc.server2.com -i nick anick newident@host.com RealName
Multiple idents in a UI don't really make or break multi ident support, but let's assume we're only talking UI here:
There's no easy way to put multiple ident boxes in the UI without completely reorganizing how mIRC handles your connections. This is not just an issue for idents, but nicknames, realnames and email information as well. Until mIRC has a registry of what info you want to use for specific networks, there is no proper UI that would solve this problem, without making it far more confusing to the user.
It should be noted that the specific suggestion of just "putting more boxes" in the ident panel will just be more complicated and more ambiguous.. If I open a new server window and edit the options for my connection there, which ident box would I change? Surely I would be editing the first box, for the active connection, no? Your proposed UI doesn't really flesh out..
A better feature suggestion would be to create a registry (as mentioned above) where you can select to use specific user info for specific networks.. that would solve not only the ident issue, but everything surrounding maintaining multiple connections. Until then, this is all handled best in script, with the /server command I mentioned above.
