Create a custom sovereign, and go into documents/my games/legendaryheroes/units folder, and find the custom sovereign file.
add these two lines into the file along with the other SelectedAbility** thingies.
Just tested it myself, and seem to work fine... except for leveling up the magic.....
<SelectedAbilityBonusOption>Life5</SelectedAbilityBonusOption> might be the only way if you want to wield both kinds of the same time. (death5 if you play kingdom) Depends on if you feel like it's cheating or not.