If I understand your idea correctly, then for two nicknames of one user, you can add 2 different entries in the "mIRC Address Book > Notify" dialog box.
The difference would be that when that user changes nicknames from X to Y, with this suggestion (only), you would not see a pair of "* X has left IRC" / "* Y is on IRC" messages. Of course, the flip side of that is that over time, you might first see "* X is on IRC" and later "* Y has left IRC", which could make logs harder to follow. Then again, one would always have the option of not using this suggested feature.
Also, it would be easier to solve this problem if wildcards were used in the nickname: «CarbonKid*» than to list several identical nicknames of different lengths.
That is unfortunately impossible as it is not supported by the base IRC protocol (specifically by the ISON
command). Keep in mind that the whole point of the notify list is that it also works when you do not share any channels with the users on there.