The WinMx username exists of two parts, the nickname and the wpn-id.
The nickname is the part you can change yourself and will not change unless you choose to do so, the wpn id however changes whenever you reconnect to the wpn which means if you state your username has not changed in two years and you think your id has not changed either you are connected to the wpn for 2 years without any interuption which will make you about the 10th world wonder.
If you add a user to your temporary hotlist and keep winmx running, the same user will be recognized even if his wpn-id has changed.
And as the name tells you, it is a temporary hotlist, adding something permanent to a temporary list .... sounds a bit useless to me.
greetz