I don't know if it's already possible (I don't think because I've only see to summon xp orbs.), but can you add these two procedure blocks please ?
1- Get Player levels (experience levels)
2- Set player level to (Write here the level to set for the player, like if I write 4, the player will have 4 levels, even if he had 1,2,10,20 levels.) (And if we want add x levels to the player we are able to do get player levels + (a number))
If you can add these two procedure blocks it will be nice !