Blizzard added health pickups that look like floating red cubes with a "+" on them. Add the Liberty Campaign dependency and duplicate one of the "Pickup (Auto) - Life +X" units and make sure that every box is checked except "Movers" and all models except "Pickup HealthX" to make it easier for you. Once the unit is duplicated, you can go into the effects to set the values "Effect: Veterancy Behavior" to the leveling behavior, and set "Effect: Experience" to the amount of experience you want to provide.
After that, you can make the unit drop it in one of two ways, data or triggers. For data, you would need to have the boss unit create the pickup unit on death, which can be done by looking at how the Hatchery spawns broodlings on death. For triggers, I'm using something like this:
Event: Unit dies Action: Set variable to random number between 1 and 100; If random number <= X: Create 1 Pickup unit at location of triggering unit