I want to add an 'add-in' or 'plug-in' to my software that requires payment. Is there anything built-in to WinLicense to support that?

If you want to use a separate license for each plugin, then you just need to put a different "License Hash" (in the Sofware panel) for each plugin that you protect. You can select the expected license name for each plugin in the Registration panel.

You can also use the same license to register all your plugins and use the "Custom Data" field to specify which plugin is supported. You can enter any text (up to 8000 chars) in the "Custom Data" field and read it in runtime via "WLRegGetLicenseInfo".

For example, the custom data field could contain a list of supported plugins names, in runtime you call WLRegGetLicenseInfo and you parse the "Custom Data" array to load the required plugins.