Adding Abilities to Units
Go into the Data Editor (F7) and under "Data Type," select "Units." Now select whichever unit you would like the caster to be. On the table to the right, find "Ability - Abilities +" and double click the space next to it to open a new "Object Values" window. Click the green X on the right to add a new line to the box on the left. Select the new line, and use the dropdown menu to find whichever ability you would like to add. Click "Ok" to close the window.
Now find "Ability - Command Card +" and double click the space next to it to open another "Object Values" window. Find a black square on the command card, and click the green X on the right to add a button. Search for the button associated with the ability you chose above, and click "Ok." Now change the "Command Type:" to "Ability Command." Change the "Ability:" to the ability you chose above (it should be near the top of the list). Click "Ok" to close the window.
If the ability costs energy, then you will need to add energy to the casting unit. Otherwise save your map and enjoy your new caster!