When using Nintex Workflow 2007, you can create constant values at the site and site collection levels which you can reference and use within your own workflows. 

You can create a workflow constant at Site Settings > Manage Workflow Constants > Add Workflow Constant 

Creating workflow constants programmatically in code, is somewhat under documented, though it turns to be quite simple; 

public static void CreateWorkflowConstant(string title, string description,
              string value, bool sensitive,
              Guid siteId, Guid webId,
              WorkflowConstant.WorkflowConstantType type, bool adminonly)
{
 var oc = new WorkflowConstant(title, description, value, sensitive,
           siteId, webId, type, adminonly);
 oc.Update();
}

Another great use of Workflow constants is to store security credentials, which you can use to call web services for instance. Workflow constants can be of the following types; 

• Credential
• Date
• Number
• String

The Credential type has a value which is stored in a particular (string) format, and fortunately there is a Nintex class which you can use for this purpose called CredentialValue. 

public static void CreateCredentialWorkflowConstant(string title, string description,
									CredentialValue cred, bool sensitive,
									Guid siteId, Guid webId, bool adminonly)
{
	if (cred == null) return;

	var serializer = new XmlSerializer(typeof(CredentialValue));
	var sb = new StringBuilder();
	using (var sw = new StringWriter(sb))
	{
		serializer.Serialize(sw, cred);
	}
	var value = sb.ToString();
	CreateWorkflowConstant(title, description, value, sensitive, siteId,
                    webId, WorkflowConstant.WorkflowConstantType.Credential,
                        adminonly);
}

And so to create a new Credential Workflow constant; 

var cred = new CredentialValue("webserviceuser", "password");
WFHelper.CreateCredentialWorkflowConstant("webserviceuser", "Credentials for calling web services",
					cred, false, site.ID, web.ID, true);

.