Impostor LOD + Proxy HLOD Tool, can't specify individual mesh?

So I’m looking to build out a little system combining impostor LODs with Proxy Geo and the HLOD system. However, I’m struggling to find a way to manually set additional meshes for use in the HLOD Proxy asset as it appears greyed out. If I manually import a LOD within the static mesh editor after saving out an HLOD static mesh, then importing a plane static mesh allows me to set a second lod for this proxy asset, but the offset is crazy as it appears the simplified geo has been merged with a pivot offset even though I’ve specified for it not to be.

Going round in circles a little - can anyone help?
Thanks!
Conrad