Cannot download DTD because of firewall

I am testing a WSDL only accessible from a high security server. When entering the wsdlv URL into SoapUI, I get an error because soapUI is unable to download the DTD referred to at the beginning of the document. This is expected www.w3.org is blocked by the firewall. 

Is there any workaround? Can I ignore that error or upload DTD files manually and then point to them in some way?