Merge object into one single slotted object?

Hello! I did research for this but nothing showed up, i'm trying to separate beds so that the bedding and frame are their separated colors, and I want them to be in the same thumbnail just like how some EA objects are in the catalog, like bunk beds and such, the only option I found was to change their protoypeID to be exact but that's not what i'm trying to do, does anyone know if this is possible and how?