Since my fp Plugin is purely mathematical, is there any way to convert its dylib from Xcode to a DLL for Windows?
At first glance, it should be possible to port it to C++ or C#, whichever is closest in syntax.
Visual Studio is the Microsoft Windows tool equivalent to XCode for Mac.
See https://msdn.microsoft.com/en-us/library/dd831853.aspx
The community edition is free like XCode.
Did you try to build the plugin for Windows with the plugin SDK examples?
Take one of the Visual Studio projects and replace the source code there with your classes?