It's possible to cache variables for Minecraft players in a scoreboard. And while it might not always be possible to use that value directly in another command, it IS possible to do tedious tests of a value by effectively asking 'is this value 1? no? is it 2? is it 3? 4?' and go through every possible option, with each one yielding a different outcome.

However, to cache the values, I first have to get them from somewhere. The ideal place to get them would be from pixelmon's built in command, /poketest

For example, consider the following:
/poketest 1 duskull

This will print out to the chat that a duskull IS the Pokemon in slot one, or that it ISN'T. But for command chaining purposes (or caching values with /execute store result score @s HaveDuskull run) poketest always outputs a value of '1,' ie that the command succeeded in running. It never outputs a '0,' if a pokemon wasn't found.

Additionally, it does not appear possible to read Pokemon data through /data get entity @s

So we can't read the duskull's level, and, worse, we can't even determine what slot it is. That cuts off other avenues for how to create this quest, because another thing we might be tempted to try is not to submit a Pokemon at all, and to instead just run the pokeedit tool on the slot that has the duskull, changing a few of its properties. But we can't run the pokeedit tool on the correct slot because we can't test what slot the duskull is in.

So how could we get around this? Well, we could create a chapter that's entirely for Pokemon submissions. Then we could put 100 quests on that chapter, each for a different level of duskull. The player then has to find the correct quest out of 100, and submit their duskull on that quest. The quest then knows what level of duskull to return to the player.

Is this realistically feasible? No, of course not. We'd have to know without a shadow of a doubt what we were doing, and then we'd have to repeat it four times, and be dead-on, with no future tweaks to the reward Pokemon ever permitted.