I just tested this out (by selecting kit challenge in the top drop down and one of your albums in the second drop down) and it put it in your album (so selecting an album seems to override the MS categories). So, it looks like the answer is 'no'. Sorry. It's been a few years since I set this up in PhotoPost, but this is starting to sound familiar. It's possible that there is a configuration setting that would allow users to add to more than one category - but I think I disabled it because of the design of the gallery software: instead of using database records to show what categories a photo is in, the program physically stores the photo in a folder corresponding to a category. This design creates heartburn for me in more ways than one... 1) if someone moves their photo from one category to another, the photo is physically moved in the file system but any image/hyperlink that points to the old photo category (folder) is now broken, and 2) if the user wants to put the photo in more than one category, it takes double the storage space because a copy of the photo will be physically put in each folder.
Sorry, that was a long-winded answer.
Nutshell - no, I don't think you can without me making a change that would impact how much storage the gallery uses. Sorry!