Well, she did explain why she was there in "Dog with Two Bones," but most people tend to miss that bit of dialogue.
Basically, she said that Moya rescued her lifepod after the command carrier imploded. The other survivors left, but she stayed on Moya to repay them for saving her.
They actually had to re-write a good chunk of The Peacekeeper Wars because the actress was having a bad reaction to her makeup. She was supposed to have a much larger role than she ended up having (several of her scenes are even played by a different actress with the voice dubbed over).