Just like Static Meshes you have a couple of ways to go about this.
- You can use the option for “Merge Physics Data”. This will take any existing collision primitives and merged them all together in the HLOD settings under Mesh Generation settings > Merge Settings > Merge Physics Data.
- Alternatively, you can open the generated proxy mesh and enable Per-Poly collision for this mesh if you’d like or re-generate collision using the auto-convex collision window, like any other static mesh. Now that we have the option in 4.14 to decimate a mesh within the editor without a third-party plugin. If you wanted to use per-poly collision instead of simple collision primitives, I would suggest taking the proxy mesh and decimating it down for a LOD for your HLOD proxy in the static mesh editor (confused yet?!) and assigning that lower poly LOD as your collision mesh with the options under the static mesh settings for “LOD for Collision”.
Definitley a good question and for susinctness can you post your question over in the Stream thread as well so that Ian and Alex make sure this is seen and hopefully covered. 
