Question on using custom spells please help

Ok, heres the thing... im new at making custom spells and i cant make them so i use them from other people and give them the credit for the spells...
Well on my RPG i want to add the blood mage with the ability of Frozen Orb.
I copy over the files, but in the original map the file trigger was set




Conditions:
(Triggering unit) Equal to Blood Mage 0000 <gen>
(on this map a bloodmage is already out on the map, i dont want that i want it so when you pick him as your hero you get the frozen orb ability)

And now that i have a new map where you start you raise a hero, how can i change that so the bloodmage(when selected) will have that option... i dont want to have a preloaded bloodmage on the map alread.

if you dont understand that...

i want to have the bloodmage ability to use f orb, how can i do that but still allow other players to select him in a building.



If anyone could please help me thanks alot