Yes, you can edit voices and save as a "user" voice. There are six screens worth of parameters you can mess with. To access simply touch and hold the voice you want to edit for 2-3 seconds, and the edit options become available.

Now, accessing a user voice to use in a style might not be so easy. A quick look and while you can change voices in a style I don't see anything that would allow access to the User bank of voices. Not saying that it can't be done but it isn't an obvious choice.