What is "winfrom", do you perhaps mean .NET?

Generally speaking, there are two options:

FME Desktop: use the .NET (or, even better, the C++) API libraries, you'll find the documentation here <FMEHOME>\\fmeobjects\\dotnet\\apidoc

FME Server: use the rich REST API, documentation can be found here https://docs.safe.com/fme/html/FME_REST/apidoc/v3/

Out of the two I'd say that FME Server makes the most sense for "secondary development", if I'm understanding you correctly.