As per school policy, you must be a fourth-year student with a cumulative GPA of at least 3.2. If so you may ask permission of the instructor to take 6000-level course. If they concur, ask the professor to send an email to the Undergraduate Registrar (Jesse Rogers jr7up) requesting they add you to the course. (An undergraduate cannot directly register in a 6000-level class.)
The CE6xxx course may be applied to your undergraduate degree as civil engineering elective, as an unrestricted elective or as another appropriate elective (please complete the Engineering Curriculum Modification Request form to document this change in your curriculum). If this is an “extra” course and you do not need the credits for any undergraduate requirement, then you may later apply these credits towards a graduate degree. Note, however, you generally cannot apply the same credits towards both your bachelor’s and a graduate degree.