You have to repeat the step: Add 2D line first, then pull to Arc. Aslo you can draw it in AutoCAD then import it to Pads using DXF in.
Maybe you can use vbscript to create it
In Layout, you just start a route, press the right mouse button, and select Add Arc. Draw a half circle, click to create a corner, then repeat Add Arc. You should be able to figure out how it works easily. Play with it, it's kind of fun. But, you can't edit this, so you have to get it right in one pass.
If you need more precision, then it's best to draw the spiral in a CAD program, and import a DXF file that you can convert to copper.