I am facing this issue where I need to insert a new field in an existing document at a specific position.

Sample document: { "name": "user", "age" : "21", "designation": "Developer" } So the above one is the sample document,what I want is to add "university" : "ASU" under key "age" is this possible?

Here's what you can do, first take the document as a dict, then we will determine the index of age and then we will do some indexing, look below:

>>> dic = { "name": "user", "age" : "21", "designation": "Developer" }
>>> dic['university'] = 'ASU'
>>> dic
{'name': 'user', 'age': '21', 'designation': 'Developer', 'university': 'ASU'}

Added the university field, now we will do some exchanging by using dic.items().

>>> i = list(dic.items())
>>> i
[('name', 'user'), ('age', '21'), ('designation', 'Developer'), ('university', 'ASU')]
#now we will acquire index of 'age' field
>>> index = [j for j in range(len(i)) if 'age' in i[j]][0] 
#it will return list of single val from which we will index the val for simplicity using [0]
>>> index
1
#insert the last element ('university') below the age field, so we need to increment the index
>>> i.insert(index+1,i[-1])
# then we will create dictionary by removing the last element which is already inserted below age
>>> dict(i[:-1])
{'name': 'user', 'age': '21', 'university': 'ASU', 'designation': 'Developer'}