By default WCF will use the security token on the calling client for authenticating it. However, you can easily supply alternative credentials (usually via some log-on dialog) as shown in the download.

WCF lets the client provide custom credentials and have the service authenticate the client by looking up the credentials in the ASP.NET membership data base in SQL Server. Once authenticated, the service can use role-based security to authorize the user against the custom roles in the data base. ...

Both Internet and Intranet applications often require a custom store for user accounts and roles. ASP.NET 2.0 provides an out-of-the-box provider model as well as a SQL Sever database just for that propose. Unfortunate, the only way to administer the credentials databases is via Visual Studio 2005, ...

This is a short sample of how to implement custom authentication and impersonation with WCF. It's based on a custom ServiceCredentials implementation, with a custom UserNameSecurityTokenAuthenticator and ad hoc Identity and Principal.