SFDC Created At is a system managed field pulled from the creation date in SFDC. If you need to set a separate creation date field through the SOAP API, you'll need to have a custom field created for this.
In case you meant setting the "SFDC Created Date" field, this can be done by passing the ExternalCreatedDate field on the MObj Opportunity mObject. There is also a ExternalCreatedDate field on the MObj Opportunity Person Role mObject.