You can find the schema.json of ‘users-permissions’ plugin at path src/extensions/users-permissions/content-types/user. the you can modify the user-role relation to ‘manyToMany’
"role": {
"type": "relation",
"relation": "manyToMany",
"target": "plugin::users-permissions.role",
"inversedBy": "users",
"configurable": false
},
and save, waiting server restart. then you can add a user to many role.
Though you can modify it, I think it is hard to manage the relationship between manyToMany, it will be a mussy maybe.
Add a new field to manage the permission maybe better I think.