AbstractSeedFromJson.php

<?php

namespace App\Migrations;

use Phinx\Seed\AbstractSeed;
use Symfony\Component\Filesystem\Filesystem;
use Symfony\Component\Filesystem\Exception\FileNotFoundException;

class AbstractSeedFromJson extends AbstractSeed
{
	/**
     * @var String
     */
    protected $jsonFileName;

	/**
     * @var Array
     */
    protected $externalData;

    public function setJsonFileName($file)
    {
    	if(!(new FileSystem)->exists($file)) {
    		throw new FileNotFoundException(sprintf('Failed to set JsonFileName because file "%s" does not exist.', $file), 0, null, $file);
    	}
    	$this->jsonFileName = $file;
    	$this->loadExternalFile();
    }

    public function getExternalData()
    {
    	return $this->externalData;
    }

    private function loadExternalFile()
    {
    	$this->externalData = json_decode(file_get_contents($this->jsonFileName), true);
    }
}

UfSeeder.php

<?php

use App\Migrations\AbstractSeedFromJson;

class UfSeeder extends AbstractSeedFromJson
{
    public function init()
    {
      $this->setJsonFileName(__DIR__ . DIRECTORY_SEPARATOR . 'uf.json');
    }
    /**
     * Run Method.
     *
     * Write your database seeder using this method.
     *
     * More information on writing seeders is available here:
     * http://docs.phinx.org/en/latest/seeding.html
     */
    public function run()
    {
        $estados = $this->table('aux_estados');
        $estados->insert($this->getExternalData())
              ->save();

    }
}

That's the sort of thing.

<?php
namespace App\Migrations;

use Phinx\Seed\AbstractSeed;
use Symfony\Component\Filesystem\Filesystem;
use Symfony\Component\Filesystem\Exception\FileNotFoundException;

class AbstractSeedFromJson extends AbstractSeed
{
    /**
     * @var array
     */
    protected $externalData;

    /**
     * @var string
     */
    protected $jsonFileName;

    /**
     * @var Table
     */
    protected $seedTable;

    /**
     * @return array
     */
    public function getExternalData()
    {
        return $this->externalData;
    }

    /**
     * Run Method.
     *
     * Write your database seeder using this method.
     *
     * More information on writing seeders is available here:
     * http://docs.phinx.org/en/latest/seeding.html
     */
    final public function run()
    {
        $this->seedTable->insert($this->getExternalData())->save();
    }

    protected function setJsonFileName($file)
    {
        if (!(new FileSystem)->exists($file)) {
            throw new FileNotFoundException(sprintf('Failed to set JsonFileName because file "%s" does not exist.', $file), 0, null, $file);
        }
        $this->jsonFileName = $file;
        $this->loadExternalFile();
    }

    protected function setSeedTable($tableName)
    {
        $this->seedTable = $this->table($tableName);
    }

    private function loadExternalFile()
    {
        $this->externalData = json_decode(file_get_contents($this->jsonFileName), true);
    }
}

class UfSeeder extends AbstractSeedFromJson
{
    public function init()
    {
        $this->setJsonFileName(__DIR__ . DIRECTORY_SEPARATOR . 'uf.json');
        $this->setSeedTable('aux_estados');
    }
}

This is how I would done it. Moved the run() to the abstract class, reducing the amount needed to be edited in the seeder.