Here's a fairly simple way to do it using "upgrades".
Create a unit with 900 max hit points.
Give it an upgrade that increases it's max health by 100.
Automatically apply the upgrade at map startup (or whenever) so that it will have 1000. To lower it's health, simply reduce the level of the upgrade.
To only have a "specific unit" receive the effect, you could make the upgrade non-global (do this by inputing "true" for 'applies to all units' within the upgrade... don't ask me why it's backwards, it just is). This makes it so that the upgrade will only apply to units that are made after the upgrade takes effect. So, you could:
1. decrease the upgrade
2. replace the unit with the same type, facing same direction and keeping it's current hp/health etc.
3. re-increase the upgrade (to ready it for future occasions).
Hope all that made sense.