This tool allows you to import Unity Packages (.unitypackage) into s&box.
When importing, there is an option that attempts to convert materials (currently only supporting the main texture and colour tint).

Extension
Also install my Mesh Splitter to enable splitting of meshes upon import.

How to use
Either right-click a Unity Package within your project and choose Extract, or use the Unity Importer > Import Package (.unitypackage) from the menu bar to import a package from outside your project.