Had a customer ask me this today. Honestly it threw me, so I wanted to put it out there for someone who has had more coffee than I have today to answer.
They have an old .NET Website Project (WSP) which has WCF endpoints in it. They also have a new "server" which is .NET API based.
Can we add WCF endpoints to the .NET API project?
Best solution in this case is to create a proxy between the API and WSP like this:
API -> WCF proxy -> WSP.
WSP calls the proxy which in turns calls the API. The proxy wraps the API response into a WCF contract and passes it to WSP.