so after updating to the most recent version (easiest way would be to use the nightly version tomorrow or so) creating a mesh like that should be possible from Python like this:
Code:
from netgen.meshing import *
mesh = Mesh(2)
pnts1 = [mesh.Add(MeshPoint(0,0))), mesh.Add(MeshPoint(Pnt(0.5,0))), mesh.Add(MeshPoint(Pnt(1,0))),
mesh.Add(MeshPoint(Pnt(1,0.5))), mesh.Add(MeshPoint(Pnt(1,1))), mesh.Add(MeshPoint(Pnt(0.7,0.3)))]
pnts2 = [pnts1[0], pnts1[5], pnts1[4], mesh.Add(MeshPoint(Pnt(0.5,1))), mesh.Add(MeshPoint(Pnt(0,1))),
mesh.Add(MeshPoint(Pnt(0,0.5)))]
el1 = Element2D(index=1,vertices = pnts1)
el2 = Element2d(index=1,vertices = pnts2)
mesh.Add(el1)
mesh.Add(el2)
from ngsolve import *
Draw(Mesh(mesh))
So if its not a too big mesh then I'd suggest to use a Python script to switch all the elements you want to curve to second order ones.