If the character already has an Intro State decider, then it should be as easy as adding a bit of code to the right area of your intro. This is normally Statedef 190, but Superman uses Statedef 191 for the decider.
In this example Statedef 191 is the "Decider" and Statedef 198 is your special intro. This state has all available intros and how mugen triggers them. To add what you want, you would just need to add the code there, and code the actual intro.
EXAMPLE:
;-----------------------------
; INTRO
[Statedef 191]
type = S
ctrl = 0
velset = 0,0,0
;ADD YOUR CODE HERE IN YOUR INTRO CODE, JUST AFTER STATEDEF 191
[State 191, Special intro vs Lobo]
type = ChangeState
triggerall = p2name = "Lobo" || p4name = "Lobo" ; <----Character's name from Lobo.def file
trigger1 = Time = 0
value = 198 ; <------- Statedef of the special intro
; The actual special intro Statedef 198 (add this code after the other intros)
;---------------------------------------------------------------------------
[Statedef 198]
type = S
ctrl = 0
anim = 198 ;<-------- Animation of the special intro
[State 198, 1]
type = AssertSpecial
trigger1 = 1
flag = Intro
[State 198, 2]
type = ChangeState
trigger1 = AnimTime = 0
value = 0