I'd like to see it like this, although this would be a pretty big change...

You can define a list of nicknames for mIRC to try, and mIRC will try each one until one is available. If none are available, then there could be a box where you can control what happens:

"If none of the above nicknames are available, mIRC should:

[x] Append random digits
[x] Append underscores
[x] Put /nick in the editbox so you can try a different nickname"

And the list could be done with a simple listbox + add/remove buttons.