To be frank, any MBA course that would accept a Diploma as its entry requirement is probably an MBA which would be a waste of money.  From a time when MBAs were a rarity to the present when they are a dime a dozen, it is very important that you find an MBA which is well recognised and respected locally and internationally before you spend your hard-earned money and time on it (see The Economist article below).  An MBA is supposed to be a post-graduate course with a degree as its entry requirement, not a diploma.  A reputable and well recognised MBA not only gives you the academic and practical knowledge for advancing in the business and management field but the networking. 

However, even then, there's no guarantee that you would progress any further or be able to utilise your MBA qualification.  I've a friend who obtained an MBA from Hong Kong University and that MBA is consistently ranked first in Asia and top 20 in the world by 'The Economist' and 'The Financial Times'.  Yet, she is still unable to progress as far as she had hoped for when she started her MBA.  In Hong Kong, there are many Hong Kong and China professionals with MBA's from top ranked business schools such as Harvard, Wharton, Stanford, INSEAD and so on.  And even though her MBA is supposedly the best local Hong Kong MBA, that pales in comparison with those elite MBAs. 

If you do find an MBA which accepts a diploma as its entry requirement, do not expect it to help you to progress further in your career.  It might even be a hinderance.  I would suggest that as you are an Admin Secretary, try taking the ICSA (Institute of Chartered Secretaries and Administrators) program.  You can join the local division MAICSA and find out what are the entry requirements and opportunities for online studies.  Its an old and very well respected international professional qualification, and accepted by many universities around the world as equivalent to a degree level course.  From there, you can progress to be a company or chartered secretary, managing the corporate requirements of a company or even eventually setting up your own company to provide company secretarial services to companies.  Try looking at other professional courses too such as ACCA or CIMA and the like.  They are probably more relevant, practical and well recognised internationally.  They are also geared towards working adults as they were originally set up to provide professional qualifications to them on a part-time or correspondence (distance education) basis.
http://www.economist.com/whichmba/nothing-...rized-employers