From there you can see all your buildings. Choose the one you would like to attach the sub-building to and click on "Edit".
Once you're on the main building's page, you will find at the bottom a section called "Link More Buildings".
Write in the box the address of the sub-building you would like to add.
A dropdown list of possibilities will appear underneath the box, you just have to select the correct address and click on "Attach".
A pop-up window will then appear, asking you which building is the main one. Click on "The One I Am Editing". Of course, if the main building is the one you just typed in, click on "The One I Searched Up".
Don't forget to click "Update" at the bottom, or you sub-building won't be saved !
There you go !
Now your main building will have a sub-building attached to it.