Converting an existing triangle-element mesh into a quad-element mesh

I try to convert an existing triangle-element mesh into a quad-element mesh. The triangle-element mesh is actually generated from n2-tri.geo, as follows (files are attached):

$ gmsh -2 n2-tri.geo

I then load the resulting mesh, n2-tri.msh, and convert it into quad elements through the geo file n2-quad.geo:

$ more n2-quad.geo
Mesh.RecombinationAlgorithm = 3;
Mesh.RecombineAll = 1;
Mesh.SubdivisionAlgorithm = 1;
Mesh.SaveAll = 1;
Merge "n2-tri.msh";
$ gmsh -2 n2-quad.geo

which results in a buggy mesh, n2-quad.msh, where many nodes have coordinates (0, 0, 0).