Merging Building Groups?

Not sure if I'm missing something, but I've got a couple different multi-shop buildings that I can't seem to combine into a single group. The architecture pre-fab shell and each shop insists on staying separate pieces no matter what I try. Some of the in game pre-fab buildings are combined into single group objects, but I can't figure out how to get existing structures in my zoo into their own easily selectable groups. There's no merge button available when I multiselect multiple buildings.
 
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