Currently, people in 10,000ft can only have one discipline and role. This is designated in their personal profiles. That being said, you can account for varying types of assignments and bill rates for one person by using project phases.
For example, Joe could be working on the QA phase of a project with a PM bill rate, then work as Technical Adviser for another phase of the project with a different bill rate. To account for different bill rates at this level, you just need to Edit Phases and change the rates.