Is it is UX appropriate to use check boxes when only when option can be selected out of a total of one option? For example, if user is has option to select an existing employee from or drop down or create a new employee, I want to add a checkbox next to "create new employee" to trigger the new employee data fields. I know standard is radio boxes if only one option is selected, but does the same hold true if one option should be selected, but only one option to select from. Hope this makes sense.

Is it appropriate to have a single checkbox on its own? For example, if a user has the option to select an existing employee from a drop down or to create a new employee, I want to add a checkbox next to the "create new employee" option which will trigger the new employee data fields. 

I know the standard is radio boxes if only one option is selected, but does the same hold true if there is only one option to select from? Hope this makes sense.