Sitefinity is based on ASP.NET and as such, it follows the provider pattern
. What this means, is that you can inherit Telerik.Sitefinity.Security.Data.MembershipDataProvider (for managing users) and Telerik.Sitefinity.Security.Data.RoleDataProvider (for managing roles). Implementing a provider that doesn't use OA is somewhat tedious, so I would recommend to you that you import the roles and users from the other system that you have to support. If that there are some changes in the external system, you will have to synchronize the items.
the Telerik team
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