You will find after looking at how categories are organized that it is difficult to conceive of a bulk upload. The parent/child relationship of each category is stored in each category's record. So it would be necessary to either know all of those relationships before hand, and their corresponding IDs, and populate your upload file with that information, or rely on the admin utility which forces the data to be entered hierarcically. The latter, by the mere nature of the data entry order that is required, drives the organization of the categories and their parent/child relationships.
All that said, I've wished for something like this for a long time, but I do not believe it is forthcoming any time soon.
-GateKeeper
p.s. You could consider a browser macro or something that can mimic data entry from a keyboard, you'll have to work with it, and it is a dedious job!
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