We always recommend that apps that are intended for App Hub submission be created through a Paid webex Account, that way you avoid future issues once a sandbox gets invalidated (usually they are only intended for trials and therefore come with an expiration date). It may happen in future that you'll lose access to your app if the sandbox expires, and apps cannot be transferred to other orgs (no cross-org support at the moment). If you plan to eventually purchase a full webex account, by upgrading your sandbox to a full webex account, then you shouldn't face any issues like such.
But technically speaking, there are no restrictions on this from a technical aspect. It is possible to create apps (including embedded apps) and submit them to App hub through a sandbox org.
