django add another field via js with one model field
You're best bet is going to be to use Javascript.  Have Javascript create the new field (or remove it) on button click.  Let the user fill in the field as they need to.  When they are ready to save you'll need to catch the submit (again using Javascript) and concatenate everything into the initial textarea field and then let it submit to the server where Django should handle it.
You'll also have to then have Javascript run on page load to check the textarea and split out the different sections of your textarea.
More or less that is how you are going to have go about it.  If you're wanting to have someone write it for you then that would be a whole other discussion. 
(I know it's easy to come along and say "why do that, that's not the best way".  I often run into constraints where the best way isn't going to work, so I try not to knock others not knowing their constraints.)
I would like to add "add another field" and "remove field" button in django admin which results in to add/remove a text field respectively. And all these field should be concatenated(separated via some character) and assigned into one model textfield. How could I achieve this?