just export the existing objects and edit it in ZM (don't move it). If you're finished, you can import it back into gp4. That way the objects always line up with the other objects.
For grandstands (or just non tarmac/verge objects) i usually import 1 part and do the rest myself (create copies). The biggest work is to split it into objects again and import it in gp4. Don't use too big objects (specially with transparathy) else gp4 will go crazy and show the objects wrong.
Don't forget to fix the normals (green stuff) and use the right texture & matrials.
When you export something from ZM it will come with .tga textures. You need .tex tga's, so you have to replace the texture names in 3d editor. I also use the existing materials, because ZM will create his own and they are always brighter as the existing materials.
You can also remove the carlightmap.tex (or something, don't know how it's called by heart), but ZM creates 2 textures which is only used for cars, so you can also delete those.
The rest is just ZM and that's easy