Default headers for all resources

Is there any way in ReadAPI 1.5 and SoapUI NG to create a default header that will be used across all resources? We have a standard set of headers (such as X-Requested-By) that need to be included with every request.

Right now I am defining these headers on each resource, but it would be helpful if I could define these at a higher level (perhaps on the service, environment, or project?)