Have a look at a Unity extension called UModeler which lets users model and prototype easily and quickly without leaving Unity.
What is more, the developer states that UModeler supports not only modeling with gizmos but also sketch-based modeling which lets users draw a 2D shape on a surface and make it 3D shape in intuitive ways. “Any polygon is automatically triangulated to be fed to a rendering pipeline even though it has holes inside which will save a lot of your time. UModeler consists of about 90 tools for modeling and UV Mapping, which will make creating, transforming and editing your model a breeze in Unity.”
Visit the following pages to learn more about the tool: