You can't do it quite like that, as HP is a property of the datablock, and you'd have to give everyone a different datablock if you wanted them to have different amounts of max HP.
However you can create the illusion of higher HP by packaging the damage function to reduce the amount of damage done
package fakeMaxHP
{
function Player::damage(this,%obj,%pos,%damage,%damageType)
{
%damage = %damage / (%this.fakeMaxHP / 100);
Parent::damage(%this,%obj,%pos,%damage,%damageType);
}
};
ActivatePackage(fakeMaxHP);
Then you just do %player.fakeMaxHP = #;