Of course, you can become membership, but I think there is a list of people waiting to become membership, you better check it, I'm not sure.
I'm membership of Atletico de Madrid, I've paid 195€ up in the north side(it's one of the cheapest places). There was a list of people waiting to become memberships in the cheapest side from last year that will be the first to choose seat after the renovation of the old memberships... I think for Real Madrid is similar..