No - you don't have to migrate your entire app to PB.Net.
Bruce Armstrong has more experience here (and I haven't tried this myself), but I think you can use PB.Net to "wrap" the call to the secure web service, and deploy it as a .Net assembly. Your PB Classic app would instantiate your PB.Net assembly as a COM-callable wrapper.
In PB.Net, create a new .Net assembly. Add an custom non-visual object, and add a WCF client proxy project that invokes your web service.
-Paul-