Yeah by the means currently in the game. I could be wrong but as far as I'm aware it works as follows.
You get primordials from the underground prison wonders. The amount you can get is limited by how many prisons there are.
You get scourge from the toll of seasons event. Which only happens once per game.
If they were added as a summon to the tier 5 tome as part of your suggestion. Players would be able to get more unless an additional mechanic is added to limit how many a player could have at one time. Like you can only use these summon spells once or only when you do not have any of those units.