The first argument of the SpellChecker initializer can be both the name of a language or an enchant dictionary:
from enchant import DictWithPWL
from enchant.checker import SpellChecker
my_dict = DictWithPWL("en_US", "mywords.txt")
my_checker = SpellChecker(my_dict)
my_checker.set_text("This is sme sample txt with erors.")
for error in my_checker:
print "ERROR:", error.word
The documentation isn't clear about this, but the code is available :)