You can register your own routes in bootstrap.php file within bootstrapDirectory (default is app/admin/bootstrap.php).

AdminRouter::get('my-url', function ()
{
    return Admin::view('my page content');
});

AdminRouter::post('category/my-url', 'App\Http\Controllers\MyController@postMyUrl');

AdminRouter::any('my-page', ['as' => 'admin.my-page', 'uses' => 'MyPageController@myPage']);

All routes will be registered with admin prefix and filter.

Important: If you want to render page content within admin interface use Admin::view($content, $title) method.