If you have an ASMX client, you can get it to call a WCF endpoint with some restrictions. You have to use the basicHttpBinding on the WCF service and the service implementation (or contract) needs to be decorated with the XmlSerializerFormat attribute.
I have encountered a problem under SSL though. I am wanting to use username/basic authentication with the service over SSL. This article makes the following reference:
The easiest straightforward way for a successful interoperability scenario is to leverage on transport-layer security. This also means that a properly configured WCF implementation can interoperate with a Basic Profile 1.0 compliant ASP.NET Web Service (ASMX) that is currently deployed via SSL / HTTPS as well as with a WSE 2.0 service or client and likewise.
WCF has a standard binding called “<basicHttpBinding>” which derives its name from the Basic Profile specifications. There is a security mode within this binding called “TransportWithMessageCredential”. You can choose either a transport or a message credentials in this security mode. Setting it to <message clientCredentialType=”UserName”/> uses Transport-Level Security (SSL / HTTPS) with SOAP-Level Username token security credentials. This is in accordance with the WSS SOAP Message Security Username Token Profile 1.0 and it implements WSS SOAP Message Security 1.0 specification for username/password (for client authentication) over HTTPS (for privacy).