You have to use a .blb for the brick. In the server.cs, you can specify a .dts for the collision only. From the Brick_ForestTree add-on:
datablock fxDTSBrickData(brickForestTreeData)
{
brickFile = "./foresttree.blb";
category = "Special";
subCategory = "Misc";
uiName = "Forest Tree";
iconName = "Add-Ons/Brick_ForestTree/foresttree";
collisionShapeName = "./foresttree.dts";
};
Inside the folder, there's forresttree.blb converted from an .obj (I assume you can do that correctly), an icon named forresttree.png, and finally, foresttree.dts. From this point, you'll have to ask Dglider or some other talented people because I'm not entirely sure how it works.