The DemandCenter CRM Integration service will always create Leads when creating new records in the CRM. There is not a configurable option that would cause the system to stop creating Leads and start creating Contacts. This is consistent with the typical Lead-to-Contact workflow recommended by most CRM systems.
However, if you would like, many CRM systems (including Salesforce, Microsoft Dynamics, etc.) will allow you to configure custom rules that will automatically and instantly convert Leads to Contacts for you. For further details, please consult the support documentation for your CRM system.