Retaining OD accounts with Active Directory

We currently have Open Directory and a domain with 4 Xserves and 350 clients (95% mac). We now have an application that must live in an Active Directory domain. Any ideas on the best way to connect the two domains and share trust/permissions on both. Windows Server 2008R2 and Leopard Server at this time. Any ideas are welcome, seem to be many solutions for adding OD to Active Directory but can't find any that will bring Active Directory onto Open Directory and retain all the users that the Open Directory has now. Thanks, Damon Posey