You can only ethically do it if you have permission.
There are many ways to do it, but in relation to BTB, you need to extract the gmt and save it as 3ds or dae then put it into xpacker. You can use 3DSimEd to get the models.
But please, make sure you have permission, most people will happily grant you permission if you ask.
Once you have permission,
* Open the .scn or model.gmt in Simed,
* Right click on the object you want to extract and select 'Edit (object name)'
* When the dialogue pops up, click 'Isolate'.
* Then up on the menubar click 'Edit' and in the 'Transforms' section click 'Center Model'.
* Then back in the menu bar click 'Export', click on '.3ds' and export to a directory.
* Then, export textures to the same dir as you exported the 3ds. (this is crucial, without the tex. in the same directory, XPacker wont be able to import the textures.)