I had to look that up to know what synchronize folders in Lightroom means. If I understand correctly, you are managing your images in the Finder (i.e. referenced library). Lightroom has a command that synchronizes its catalog with your folders. When you add new images to a folder that was already imported into Lightroom’s catalog, the synchronize command will import those new images to make the catalog and the folder contents match. Did I get that right?

Assuming yes, only thing I can think of in Aperture is to click the import icon, navigate to the folder in question in the import browser, and select the project in the library that matches the folder selected. Check the “Do not import duplicates” checkbox in the import settings. You should only see the new photos now and can click the import button to bring them in - effectively synchronizing them.