import json
users = """
{
"users" : {
"23423424" : {
"nums" : [14, 3425, 33, 2],
"first_name" : "Alex"
},
"234442" : {
"nums" : [33, 1, 99, 42],
"first_name" : "Gosha"
}
}
}
"""
my_dict = {
"777" : {
"nums" : [2131, 666, 91, 2],
"first_name" : "Max"
}
}
dict_users = json.loads(users) # получаешь словарь из строки джейсона
dict_users["users"].update(my_dict)
print(dict_users) #{'users': {'23423424': {'nums': [14, 3425, 33, 2], 'first_name': 'Alex'}, '234442': {'nums': [33, 1, 99, 42], 'first_name': 'Gosha'}, '777': {'nums': [2131, 666, 91, 2], 'first_name': 'Max'}}}