Before conversion, make sure you have created SourcePrefabTreeAsset, it is used to distinguish your unitBlock type.(You can create it by right click in the project panel "Create/RMM3D/TreeAssets/Source Prefab Tree Asset")
At "Convert Fbx" tab. You can see the SourcePrefabTreeAsset you created, drag and drop your source prefab to the treeview box on the left.
Up on the treeview box, you can choose what type of unitblock to convert.
After clicking the convert button, you can find the converted unitblock in the output directory.
Of course, you can also change the output directory in the settings tab.