It is possible to create a Custom Action that would create a new User and set its fields according to what was predefined in the Custom Action. For example:
Please note that this is not a complete Custom Action, but just a part of it (you'd have to create a separate conditional action list for every possible Role choice). Video of this Custom Action: https://www.screencast.com/t/byT3tXUMAQi
Please let me know what you think about this solution.