For future reference, the best place to post something like this would be the thread for those characters, or the DTW thread, if it's a problem/question specific to the program.
Anyway, what do you mean it becomes compressed? DTW has no compression feature. If you import a file that's 400 Kb larger than the original, the new ISO would then be 400 Kb larger. (The next version of DTW will behave a bit differently, and have new options for controlling the size of the finished ISO.)
I don't know how to convert it to use the Neutral slot, since the model of the DAT file is changed as well. Maybe the model data could be copied over directly, in a hex editor, but I kind of doubt it since there are probably other bits that link up to it differently (i.e. the new structures would be at different offsets). The person to ask would be
@zankyou, but I assume he would find it easier to just re-convert the costume over the other slot.