Automatically update Sales Rep on Qualifed Leads sent from an External System through web services

We have an external system send leads to CRM.  Mostly they are qualifying leads, but we would like to have some leads come over in a Status of Qualified.  We are running into issues with the Sales Rep field being required for a lead to Qualifed lead to be succesfully inserted into CRM.

Specifically:

For leads our external system includes information so the lead is assocaited with a CRM account when inserted.   We have been leaving the sales rep field blank and CRM updates the Sales Rep to the the owner of the account.  Qualified Lead require a Sales Rep on the lead.  We have tried hard codeing a user and can successfully insert a lead that is qualified, and the lead is assocaited to an account when inserted.   The Sales Rep field is not updateing to the owner of the account.   Is there a way to do this automatically?  Ie. workflow using join