converters.php

<?php

class FooConverter
{
    public function convert($value) {}
}

$app = new Silex\Application;

$app['converters.foo'] = function($value) {
    $converter = new FooConverter(); // or could pull from container if dependencies are required
    return $converter->convert($value);
};

$app->get("/foo/{foo}", function($foo) {

})->convert("foo", $app['converters.foo']);