Machinations Toolkit 3DS2PFB Converts 3DS MAX (tm) 3DS files to RADIANT prefab brush PFB files.
HMF2 Radiant does not support brushes that have any concavity so even though 3DS2PFB will convert meshes with concavity the resultant brush will not successfully open in HMF2 Radiant.
Open a command prompt at the folder that contains 3DS2PFB.exe and type "3DS2PFB" followed by the location of an existing 3DS file and the location you wish the resultant PFB file to be saved to e.g. "3DS2PFB my3dsfile.3ds mynewpfbfile.pfb" the program will then run giving you information about what it's doing. Your new PFB file should now be openable in HMF2 Radiant.
Available on the tools page.