To See below on how to convert an existing user to a salesperson.
Add the user in the Salespeople Drop Down.
Change their Rights Profile accordingly in the Users Tab.
Configure Salesperson / Manager / Action Item Config tab.
Attach user as Salesperson in Human Resources Employee Manager.
See checklist for a New Employee Setup. Although the user is not a new employee and most of the things on this checklist will already be completed, this is a good reference on what settings you may need to setup and/or change.