There is an action known as Unit - Set Max life of Unit, but that will not have the desired effect you're looking for. Instead, find the Item Ability that gives life +, can't remember what it's called but I think it's something in the lines of Item Ability Life +. Adding this to your unit will increase its max hp. Simply use Unit - Add Ability to Unit, and then remove the ability if you don't want him to have the bonus life.