There's no plans to allow for extensions other than custom blocks, custom textures and Lua scripting. There's technical limitations which keep features like the one you describe to be added. Asset Forge works in a way where users place blocks (meshes), but those blocks cannot be edited - that's not supported due to the save file structure (it doesn't save mesh data).
If you'd like to create your own blocks, or advance edit them, do use Blender. I can add these features in Asset Forge but they'll never be as good as Blender's, so it doesn't make sense to compete with that.