Filling Word templates using web services is a standard tool. It can be enhanced with a custom logic using additional fields and BAdI implementation.
Regarding your main question. You have to use GDCOIC as a business object, "Virtual Object for IC Web" as a main object (GDCOIRoot) and CRMIC_DEFAULT as a component. This is described in 3.6.3 Defining Web Services chapter of mentiond C4A building block. And you may refer this discussion too: Re: Template designer and WebService