And here is how to do it with data only. Give the units to be converted a Buff behavior with the Combat - Damage Response - Handled being set to an Apply Behavior effect. You will also need to add a validator of the Unit Compare Vital type to the Behavior - Validators (Disable) and a cooldown in the Combat - Damage Response - Cost - Cooldown field. Set the Combat - Damage Response - Chance to 1
The Apply Behavior applies a Buff to the attacking unit which has Combat - Damage Response - Handled being set to an Apply Behavior effect, a Combat - Damage Response - Chance of 1, the Combat - Damage Response - Location set to Attacker and a low duration in the Stats - Duration field.
The second Apply Behavior effect has two validators, the first is a Unit Compare Vital type like the one used by the unit to convert except set to target under the Validator - Unit - Value and the second is a Unit Compare Behavior Count which counts the first Buff to ensure that only the unit desired can be converted. The final Buff applied has the Behavior - Player - Value set to Caster and the Effect - Effect - Initial set to Zerg General - Post Morph (Healing Effect)
All Buffs have the hidden flag checked under Stats - Flags
Well in short the unit to convert has a buff with a damage response on defense that gives a buff to the attacking unit that has another damage response on attack that applies a conversion buff like the infestor's neural parasite which then also applies a heal effect to reheal the unit back to full health on conversion.
The first validator is like the trigger method to only allow the unit to be converted when under a certain amount of health while the second and third purely make sure that only the unit with the buff that enables conversion and if it still has the right amount of life can be converted.
Next to that you cannot get more precise than I have. If you really want to know more details about individual fields, please feel free to look at the wiki on this site.