Is it feasible to manually draw from scratch a 2-layer PCB inside HFSS
You can do either. If you build the geometry directly in HFSS it will be easier to adjust it, do parametric sweeps, etc., to optimize the design.

If you import from a CAD tool it will be easier to generate the manufacturing files when you're done. 

One way: build and optimize the design in HFSS. Then rebuild it in CAD. Then import from CAD to HFSS to make sure you got the CAD design right.