you can draw any shape in altium. example you can use "Place>Full circle" command for circle. You can make Arc, straight line, etc., Make sure you have selected a net from the object property menu(By double clicking the object). Another way, you can import 2D outline from anyother software in DXF format and pour "polygon" inside the layout.
Draw any closed loop 2D outline (Example : Full circle), select the entire object & follow this. Tool>Polygon Pours>Define from selected objects