huh? I do it all the time dude... it's not good to say to someone that you're sure that they haven't tried it yet...
here's what you need to do...
Art - model file = change this to your custom tree
Also be sure that Variations are set to Zero, or else you would surely have green boxes
and some notes:
1)make sure you imported the custom texture too if the model uses one
Now if it does, and you have already saved the map without the texture before, the WE won't recognize the texture anymore so here's what you need to do
1)delete the model file (and the texture if you imported it too)
2)save the map
3)restart WE
4)open your map
5)import the model and the texture and be sure to set the texture to its correct path
6)save the map