Add a Spark of Legend in-game, then fill the new slot with another one via modding. Return to the game and fill the new empty slot with a cheap enchantment, then mod that to another Spark of Legend. Repeat until you’ve got enough slots full with Sparks of Legend, then change all of them to enchantments of your choice. (E.g. for criticals: one Shard of Dilaera gives 3% CritHit chance, so to get 100+%, you’ll need at least 34 slots.)
If you need enchantment IDs, consumable IDs and item type codes, I’ve just posted them in tables over here.