X-Men Character Pool is a mod for XCOM 2, created by arkaeyn.
It adds bunch of X-men as characters, including Cyclops, Iceman, Beast, Havok, Legion, Wolverine or Storm.
To install use Nexus Mod Manager or alternatively:
1) Unzip to C:\Users\YOURNAME\Documents\My Games\XCOM2\XComGame\CharacterPool\Importable or equivalent
2) Open Character Pool from the main menu
3) Import all or selected characters
4) Packs can be added when you want more. For example, start the game with the X-Men Classic pack, when it starts running out of characters, import the New Mutants and they should start showing up
5) To have ONLY these characters appear, edit your config file. See how here.