CNE is a certification, not a postgraduate certificate. As such, you have to meet the requirements to qualify to be certified.
This certification and its requirements to test can be accessed here and I have cut and pasted the exact requirements below. Note that though your current education is impressive in your field, you are not qualified to sit for CNE certification. You would be if you take more graduate level courses, however, in nursing education, examples which are listed below:
Certification for Nurse Educators (CNE)
Option A: Must meet criteria 1 & 2
-A currently active, unencumbered, registered nurse designation in the country where currently practicing as a nurse educator.
-a master's or doctoral degree in nursing with a major emphasis in nursing education or
-a master's or doctoral degree in nursing plus a post-master's certificate in nursing education or
-master's or doctoral degree in nursing and nine or more credit hours of graduate-level education courses*
Examples of acceptable courses include: Curriculum Development and Evaluation; Instructional Design; Principles of Adult Learning; Assessment/Measurement & Evaluation; Principles of Teaching and Learning, Instructional Technology
Note: Graduate-level research or statistics courses do not count toward this requirement
Option B: Must meet criteria 1, 2 & 3
- A currently active, unencumbered registered nurse designation in the country where currently practicing as a nurse educator.
- A master's or doctoral degree in nursing (with a major emphasis in a role other than nursing education).
-Two years or more employment in a nursing program in an academic institution within the past five years.
All eligibility criteria for initial certification must be met at the time of application.