Adding additional HTTP headers for SOAP client

We need to use SOAP API for some webservice. I created the request/response & operation classes via the SOAP wizard. Prior to making the invokeMethod, 

..Adapter.InvokeMethod

i need to set a custom HTTP header as part of the SOAP call. Can you please let me know how i can set that. That Adapter ( EnsLib.SOAP.OutboundAdapter) class does not have methods for setting HTTP headers.

Thank you,

Ravi