MUI_UNPAGE_CONFIRM is not a nsDialogs page!
To change this dialog you can use the ChangeUI command. You can also add controls at runtime but only by using the system plugin:
Function un.ModifyUnConfirm
FindWindow $1 "#32770" "" $HWNDPARENT ; Find inner dialog
System::Call 'USER32::CreateWindowEx(i${__NSD_Label_EXSTYLE},t"${__NSD_Label_CLASS}",t "Testing!!!",i${__NSD_Label_STYLE},i 50,i 100,i 400, i 25,i$1,i0,i0,i0)i.s'
Pop $0
SendMessage $HWNDPARENT ${WM_GETFONT} 0 0 $1
SendMessage $0 ${WM_SETFONT} $1 1
SetCtlColors $0 "" ${MUI_BGCOLOR} ; This is the wrong color to use...
FunctionEnd