This is a tough question with out more information. There are a lot of restrictions placed on SBS such as:
Windows Small Business Server restrictions
Windows Small Business Server provides a cost-effective suite of programs for small businesses that have 75 or fewer users or devices. To help you prevent unapproved network installations and to help you protect your networks against viruses and other attacks, Microsoft has placed the following restrictions on the use of Windows Small Business Server:
• You must install all the component server programs in Windows Small Business Server on a single computer. No component may be installed on a separate computer. If you want to run one or more of the component services on a computer other than the Windows Small Business Server-based computer, the other computer requires separate licenses.
• You can connect a maximum of 75 users or devices to a Windows Small Business Server-based network. This requirement limits the number of users or workstations that can access one or more of the component server programs.
• Windows Small Business Server is a single-domain solution that is not intended to be integrated with other Windows-based domains. You are not permitted to establish explicit trusts to other Microsoft Windows NT-based domains or to Active Directory directory service domains. Also, Windows Small Business Server does not let you create child domains. The computer that is running Windows Small Business Server must have all operations master roles.
• The following features that are available in earlier versions of Small Business Server are not available in Windows Small Business Server:
• Microsoft Shared Modem Service
• Microsoft Terminal Server
• Windows Small Business Server supports up to two physical CPUs and up to four virtual CPUs with hyperthreading.
Note Earlier versions of Windows Small Business Server support up to four physical CPUs.
• You can have other domain controllers and member servers in your Windows Small Business Server-based network. However, the computer that is running Windows Small Business Server must be set up as the root domain controller of an Active Directory forest. This requirement may prevent the computer that is running Windows Small Business Server from serving as a corporate branch office server or as a department server of a large organization that is based on Active Directory or on Microsoft Windows NT 4.0. You cannot add Windows Small Business Server to an existing domain. You must install a Windows Small Business Server-based domain and then add additional domain controllers or member servers to the domain.
• When you run Exchange 2003 on a Windows Small Business Server-based network, Exchange 2003 cannot be part of a larger Exchange 2003 organization. Because Windows Small Business Server is installed at the root of the Active Directory forest, you cannot install Exchange 2003 components that belong to an existing Exchange 2003 organization.
If all of that is ok and you are aware that this has to be A STAND ALONE DOMAIN CONTROLLER. You WILL NOT be able to add another DC and you are limited to 75 users. You can migrate your users and ad structure as well as your scripts and policies but there is no automated way to do this.
Let me know ify ou need more help.