Yes, it is possible, but the better question is would they? The CA's reputation is their life blood. Why would they give a trusted signing authority to another CA that might not hold up to their standards (and could be a direct competitor in the future.) To the best of my knowledge, no CA offers this kind of service, nor do I see any reason why they would consider doing so in the future. Apparently some CAs do allow this, but they have extensive requirements for the security of the sub certificate and charge a very large amount for the privilege.
