1-open a map in hmt
2-go to pc models,
3-export the model of what ever u want to change, to somewhere u can easily find it
4- open gmax and import the file into it. (if u dont have the .ogj plugin, google it)
5-do your modelling
6-export it as a .ogj file.
7- open the same map back in hmt
8-import the file back to where it belongs
9-load game and enjoy.