Pixelmon is based on Gen 6 (XY/ORAS)
Pelipper with the drizzle ability (and +10 sp atk I must add) wont be added until the team decide to add Gen 7 mechanics (based on last time that will be when gen 8 comes out).
It's not like they don't have plenty to do between now and then with gen 4, 5 and 6 still needing their dexes filled.
There is a mod you can get (it's server and client mod) called nbtedit. With this you can change any pokemons ability to whatever you feel like. So on my server (big up InitiaNova) I have a lot of alolan pokemon like drought torkoal and sandstream gigalith etc.
However that mod is an admin thing not only will you need to ask your owner to add it... you wont get access to it yourself