How can I add one enchant level?

you can do it, but it will need multiple command blocks, first of all, you will use testfor for example

/testfor @a {SelectedItem:{id:"minecraft:iron_sword",Damage:0s,Count:1b,tag:{ench:[{id:20s,lvl:1s}]}}}

that's if you want it for an item, but if you want it for a piece of armor try this

/testfor @a {Inventory:[{id:"minecraft:chainmail_chestplate",Damage:0s,Count:1b,Slot:102s,tag:{ench:[{id:4s,lvl:1s}]}}]}

after using the testfor in a repeat command block that is always active and nonconditional then put in front of it a chain command block that is conditional and always active and then write inside of it

/replaceitem entity @a slot.armor.chest minecraft:chainmail_chestplate 1 0 {ench:[{id:4s,lvl:2s}]}

and if it had level 2 repeat the steps mentioned up, just replace the first command(for the armor) enchantment from 1 to X(the level you want) and the second command from 2 to X+1

wel idk if this helps but this is the command for 1.14.4 /replaceitem entity @p armor.head chainmail_helmet{Enchantments:[{id:protection,lvl:1}]}