Not supporting custom fields was a deliberate decision: http://www.joelonsoftware.com/news/20020912.html

"But the dilemma for us is that many customers are evaluating bug tracking software and they consider the lack of custom fields to be a major weakness in our product.... But it's still rude of me to tell customers that we don't have that feature for their own good, even though it usually is, and we're losing some sales because of it."