You can't necessarily "add additional keys to existing values", unless you simply meant add additional key/value pairs to the existing dictionary. Here's some examples of adding elements to a dictionary:
>>> d = {'a':1, 'b':2, 'c':3}
>>> d['d'] = 4
>>> d['a'] = [ d['a'], 12, 13 ]
>>> d['e'] = []
>>> d['e'].append('foobar')
>>> d
{'a': [1, 12, 13], 'c': 3, 'b': 2, 'e': ['foobar'], 'd': 4}
>>>
Reputation Points: 355
Solved Threads: 292
Veteran Poster
Offline 1,102 posts
since Jul 2008
Unsolved 
