Need Urgent help to update Opportunity Product

 Hi All,

        Need an urgent help.We don't have any External Unique Id field in Opportunity Product.So foe Existing Opportunities We don't have that Id .Now we need to Update all those records,But for Updating through import export tool External unique id is mandatory.

So we have tried through data loader,There also getting an error log that External Unique Id needs to be mapped.

Is there any work around to Update Opportunity Product without External Unique Id through Webservice.If yes,Please share.

         Thanking you in advance.

Best Regards.

Souvik