Textbox with addon selector

My users need to input a number. For this, I would like a control that is a textbox for the numerical input and a section on the end of the textbox that indicates a default unit of measure, similar to Bootstrap's appended inputs. Also, the user can click the addon section to change the unit of measure.

Is this a good idea? Is there a control out there that does this already?

Update: I did a quick mockup in jsBin that shows what I'm thinking. http://jsbin.com/ivafag/1

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