I want to add this skill can I?

ok, you know that orc guy with the flame thrower
I want to make it so once he attacks it puts immolation on the other people for like 5 seconds but i want the fire to like 30damage/sec

I don't think there is an easy way to do this. There is no trigger to add immolation to a units ability list.

You can try the following triggers to emulate the damage..

<Trigger #1>
Event:
Unit - A unit owned by <Player> is attacked

Condition:
Unit-type of (Attacking unit) Equal to <Desired Orc Unit>

Action:
Countdown Timer- Start <Timer #1> as a One-shot timer that will expire in <Immolation Duration> seconds

Trigger- Run <Trigger #2> (checking conditions)

<Trigger #2>
Event:
None

Condition:
Remaining time for <Timer #1> Greater than or equal to 0.01

Action:
Unit Group - Pick every unit in (Units within <Radius> of (Position of (Triggering unit))) and do (If ((Owner of Triggering unit)) Not equal to <Player>) then do (Unit - Set life of (Picked unit) to ((Life of (Triggering unit)) - <Damage>)) else do (Do nothing)

Wait <Delay> seconds

Trigger - Run <Trigger #2> (checking conditions)


Modify the following variables to customize the trigger:
<Trigger #1> - Name of First Trigger
<Player> - Player who will be controlling the immolated units
<Desired Orc Unit> - The orc with the Flame Thrower, or any other orc you desire to catch people on fire
<Timer #1> - The name of the Timer used to repeat the trigger
<Duration> - How long the immolation will last after being attacked
<Trigger #2> - Name of Second Trigger
<Radius> - How far away the immolation will burn other units
<Damage> - How much damage the immolation does per delay cycle
<Delay> - How much time passes between immolation burns

You can probably emulate the immolation aura with another trigger to play a special effect over the immolating units, but I do not have the string values of the special effects yet to help with that.