I ended up adding these fields by essentially, in a plugin, extending the UserItemBeanAbstractProfileProviderImpl class to create a new Provider added to the userItemBeanBuilderProviders, which I've overriden in the spring.xml file.
I found this post in a Google search because this is exactly the customization that we're looking to make. Would you be willing to share any tips on how you made yours work or possibly share your code? My coworkers have put some effort into developing a plugin, but had trouble due to lack of documentation. Any help you could offer would be greatly appreciated!