After a quick max change, I noticed there is a holster that is a skinned object so it means I can dump in another skinned object. :P
So deleting the body from the psyker and leaving the helmet in there, I should be able to put the Death cult assassin underneath without stress, I believe. :P Body sync should do it good. :)
Since I have been struggling to get the mounted warp beast to work properly I thought I should try something that might make it seem easier to figure out. :P