The method that bonus systems use is to add item mana/health bonus ability. Give the abilities levels. Make level 0 give 0 bonus. Make the other levels give negative bonuses.
To enable negative values, got to file -preferences and tick the box there.
Then select a data field and press shift + enter to edit the value. Enter a negative value.
Add the ability to the unit, set it's level to something else than 0 and remove the ability right away. The unit will get bonus max hp/mana.
You could make lvl 1 give 1 bonus, lvl 2 2 bonus, lvl 3 4 bonus, lvl 4 8 bonus, lvl 5 16 bonus so the increase can be any value, and you can easily set it to the correct value by using a loop and adding the ability, setting it to the correct level and removing it.
For example +100 hp:
Add, set to level 7, remove (+64)
Add, set to level 6, remove (+32)
Add, set to level 3, remove (+4)
^You can use a loop with that.
And then the unit has +100 max life.