Use the EnvironmentSettings component, it can be added from VisualStudio Toolbox. In the DatabaseLogin event you may pass own connection string to the connection object that is defined in the report.
private void report1_DatabaseLogin(object sender, DatabaseLoginEventArgs e)
{
  e.Connection.ConnectionString = my_connection_string;
}
Comments
Use the EnvironmentSettings component, it can be added from VisualStudio Toolbox. In the DatabaseLogin event you may pass own connection string to the connection object that is defined in the report.
See the class reference topic about this event.