Eg:Export an existent mesh and open it with 3DS... add a .obj (don't not attach to the first !) and export in .obj format after import in .GR2 editor
But,you need to make the same with the "exported"_AO.obj to have the same vertices/faces number !
Edit:don't forgot to see if the UVW are created
if you want to modify a mesh the best is to open the 2 files in same time ... (the "exported".obj and the "exported_AO.obj)