make an ability of an item ability(khadgar's gem of health)
edit it the way you want, say 500 HP, and use the unit - add ability (500hp ability) to (the unit you need_variable)
or if it's an aura, check out the Custom Auras thread, and use same tactics as described...
How about, instead of giving it 50% more hp, have it take 33% less damage? I believe basing an ability off berserk and leaving the attack at 100% while setting 'damage taken' to 0.66 would do this.