Venture Capitalist Scott Weiss has a blog post discussing the merits and challenges of promoting from within, along with some sage advice from his father. The recommendation was never to take a top salesperson off the floor to be a people manager because most will not work out.  That my friends is an old school endorsement for dual career paths.
His Dad’s advice is a bit broad for my taste, but it does showcase a fairly common challenge: a lack of equally attractive career paths (financially or otherwise) can contribute to people being drawn to a role for the wrong reasons. As an organizational leader, you don’t want to lose your best people but there will come a day when someone declares they want to try out people management and – despite your intuition telling you otherwise – you decide to accommodate their request. 
Scott gives some good advice for how to properly set things up, and I’d also add that this is a great example where an external coach (usually called an Executive or Leadership Coach) would be quite helpful. Typically a coach would be sponsored by the organization and would receive information on key priorities, strategies, values, in addition to access to key people with whom the coach could speak so as to understand the client’s strengths and development areas.  The conversations between the client and coach are completely confidential although it’s quite reasonable for the coach, client, and the client’s manager to meet every three months so the client can give updates and share insights with his/her manager.
A leadership class is helpful, as are books, but in my mind nothing beats a coach’s ability to help the client understand what’s behind her/his interest in management, key strengths, and development areas relative to explicit leadership competencies (if they exist) or a standard listing from any number of 360 tools. Since coaching is a partnership driven largely by the client, it’s highly customized and experiential, both of which will help drive skill development and behavioral change faster than a class or book.
So go ahead and encourage people to follow their dreams, but be prepared to help them fully understand what is expected, and prepare your organization and the person for success by investing in their development. I’d be delighted to help you as a coach, and with determining what leadership means in your organization.