Its is a good practice to cast from an interface to some concrete class when needed?

I'am developing a small system and i developed the classic generic repository. For now, i have the following architecture for my DAL.

public interface IRepositorio<T> where T : class
{
    T Get(long id);
    long Insert(T obj);
    bool Update(T obj);
    bool Delete(T obj);
}

public abstract class Repositorio<T> : IRepositorio<T> where T : class
{
    public IDbConnection Connection
    {
        get
        {
            return new SqlConnection(ConfigurationManager.ConnectionStrings["DBFila"].ConnectionString);
        }
    }

    public T Get(long id)
    {
        //...
    }

    public long Insert(T obj)
    {
        //...
    }

    public bool Update(T obj)
    {
        //...
    }

    public bool Delete(T obj)
    {
        //...
    }
}

My concrete repository looks like this:

public class FilaRepositorio : Repositorio<FilaRepositorio>
{
    public FilaRepositorio() 
    {
    }

    public void SomeCustomMethod()
    {
        // Some custom method
    }
}

I am also using Simple Injector to follow the IoC and DI patterns, for this reason, when i try to call "SomeCustomMethod()" i dont have access to it (obviously). Look:

public class Processador
{
    private IRepositorio<FilaModel> _repoFila;
    public Processador(IRepositorio<FilaModel> repoFila)
    {
        _repoFila = repoFila;
    }

    public void Processar()
    {
        _repoFila.SomeCustomMethod(); // <-- wrong

        ((FilaRepositorio)_repoFila).SomeCustomMethod();// <-- works
    }
}

Given this i have some questions:

• Is a good or acceptable practice to make that cast (FilaRepositorio)?
• If its not a good practice, how to write good code for this case?