The Locust unit is just like any other unit, you modify it the same way you would a Footman or Grunt.
So to match Exorcism you would change the Attack Type to Hero and the Base Damage to 52 and the Damage Sides per Die to 6.
Some more information about the Locust Swarm ability:
Data - Damage Return Factor - tells how much % of damage done by each locust is returned as life for the caster. I think 0.75 means a Locust will return 25% of it's damage dealt.
Data - Damage Return Threshold - describes maximum amount of damage, single swarm unit can deal per round (it has to return to the caster to be able to cause any damage again).
Data - Max Swarm Units Per Target - maximum amount of locusts focused on single unit.
Data - Number of Swarm Units - total amount of units summoned.
Data - Swarm Unit Type - unit-type of summon.
Data - Unit Release Interval (seconds) - the delay between summoning each new locust.
Stats - Area of Effect - tells how far the locust can move away from the caster, it immediately returns if distance between it's position and caster is greater than given value.