I was awarded PhD in 2020 by a University as per regulations applicable at the time of my admission where Coursework requirement was only 4 credits ( One course) + Comprehensive Exam + Publications + Thesis + Viva Voce etc... To get equivalency on par with US university PhDs which have more coursework (say 32 credits of coursework), can I now do additional coursework from other universities and add additional 28 credits of coursework to my already awarded PhD by any means? or Should I show them as postdoctoral courses? How to add value to my PhD with these courses done after PhD?
The US Ph.D. tradition is a bit sui generis. In order to get admitted to a Ph.D. at an Indian university, you always need to have an M.S. degree in the subject. In a US type Ph.D., you can get admitted into the program right after finishing the B.S. (which does not have to be in the same subject matter). You can equate the Indian Ph.D. program with the US type Ph.D. program after the comprehensive examination.
The reason that you might feel the need to update your Indian Ph.D. is that US (and Canadian, ...) degrees have usually a higher reputation, because they demand more from a thesis. In Academia, you could upset with good publications, e.g. in Computer Science at highly ranked conferences to which US Ph.D. candidates submit, (which is of course very difficult since you graduated from the program and no longer have easy access to a mentor.) Additional course work will not be impressive and rather seem an indication of a fundamental mis-understanding. In Industry, it is your skills that matter and more academic courses might not be the best way to gain additional skills.
How to add value to your Ph.D. is an important question to you, but the answer will depend very much on your goals and circumstances.
There would be little point in it actually.
Note that doctoral students in US normally start with only a bachelors and the coursework is to enable them to pass comprehensive exams. They likely need advanced coursework beyond the bachelors. It has little additional purpose other than to assure a broad in-field education.
But you have fulfilled the purpose already.
It might also be difficult to do unless you want to enroll in another degree program, though some universities will let you enroll in individual courses.
I'd spend the effort on research in field, rather than courses, unless a specific course would help in the research.