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.
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.