Warning : The migration will only work in PC or Labtop 

If you beat King paradox (atleast once), you can go to basin town 

There are some conditions you need to complete in order to start the migration

Prerequisite in-game:

1) You need to complete the main storyline

2) You need to defeat King Paradox (Atleast once) in the realm of Arceus and then go to Basin town, enter the house on the left side of town and talk to the assistant in order to start the migration

Technical Prerequisite:

1) Pokemon from other fangames needs to be original and from Generation 1 to 8 only

2) Other Pokemon fangames needs to be made in essentials 17.2 (Sadly not all fangames are compatible)

3) You need to put the save file from the other fangame into 2 folders

(1:Migration folder) (2:Graphics/Pictures/Migration) and name it Game.rxdata (if it doesn't)

before talking to the assistant again

This is the list of fangames that are initially compatible to transfer from

Pokemon Void (just rename it to Game.rxdata)

Pokemon Green Remix

Pokemon Nimbus

PS_The First Journey

Pokemon Titan

Pokemon Infinity

Pokémon Infernal Red / Natural Green

Pokémon Divine Purple

Pokemon Looker

Pokemon Rose Gold

Pokemon Zeta

Pokemon Mega Adventure

Pokemon Sunday

Pokemon Kronos

If you want to try this method with other fangames , do it at your own risk