I would say no, as the characters you are making money off of are not your own, and Tapastic should probably be a place for original ideas-- not to mention that there's entire legal hurdles they would have to go through because not only would you get into trouble for making money off of anothers' work, Tapastic can get into trouble for hosting it and allowing it.
labelling something a "Doujinshi" is not equal to getting legal consent from the creators to make money off of their work.
I would leave it unsupported and work on getting an original comic supported, using Doujinshi's as a means to catch certain demographics' eye but that's all.