Could you please answer the question which Daniel asked? There should be no problem to add custom providers following our documentation. As Daniel noted if you send us your code we will get a better picture of the problem.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items