Hi,
First of all, I suggest that you make sure that you correctly configure your SAP web service, following the guidelines stated in this link:
http://www.riyaz.net/sap/xipi-a-guide-to-using-sap-xi-soap-adapter/113/
If you still have an issue trying to consume this SAP service in your BizTalk orchestration, the next step that I recommend is that you try the solution proposed in the following blog and see if that resolves your issue:
http://seroter.wordpress.com/2007/04/02/consuming-web-services-in-biztalk-without-web-reference/
Let me know how you make out on this one.
Good luck.