Under adding a file, you can submit maps under "Use Map Settings map" and "Multiple Use Map Settings Map" but once the file is created there doesn't seem to be a way to add another file to a download, although you can change the file itself. Just that a person asked me how they would attach a sequal to a map they had previously made without getting rid of their original map, yet still having them in the same download.
Guess what, you're the DLDB Keepers. Delete the old one and accept the new .zip that someone wants to submit into the DLDB.
Can't the submitters just modify the maps themselves to a .zip file?
If it's a sequel, isn't it a new map entirely?
The setup for submissions is gay though, yeah.