Import file into Zbrush and Export Normal map from Zbrush

 

1.  Unwrap UV properly before export OBJ file from Maya

2,  Import OBJ file into Zbrush

3.

1) Import one part

2) got to subtool

3) insert a layer

4) import another part

Export Normal Map from Zbrush to Maya

1

2.

3.

extra info:

Delete unused surface from file