According to the source code for sklearn.feature_extraction.text, the full list (actually a frozenset, from stop_words) of ENGLISH_STOP_WORDS is exposed through __all__. Therefore if you want to use that list plus some more items, you could do something like:
from sklearn.feature_extraction import text
stop_words = text.ENGLISH_STOP_WORDS.union(my_additional_stop_words)
(where my_additional_stop_words is any sequence of strings) and use the result as the stop_words argument. This input to CountVectorizer.__init__ is parsed by _check_stop_list, which will pass the new frozenset straight through.