You should likely start with a composite (one to many) relationship class between your fc and your look up table.
You should be sure to add the ground floor postal code to the look up as an alternative for that building, and not just have it in its own field as primary key.
(just to be less confusing, I would give the buildings a unique ID other than one of the postal codes)
You should be able to access all the associated postal codes for a building through the relationship.
If that is not enough, and you really want multiple coincident polygons, you can use the relationship to export the features by their floor attribute: a fc of all ground floors, a fc of all first floors (second floor in the US) that have postal codes, and so forth.
Then append the separate fcs together.