Edit System Dictionary on Mac OS 10.8
Warning: be careful about editing the system dictionary as it is easy to get an unusable dictionary. You also run the risk of having the updated dictionary over-written by subsequent OSX updates.
To edit the system level dictionaries, there is a helpful Q&amp;A that tells you where to find the dictionaries: superuser.
In summary:

~/Library/Spelling/LocalDictionary
The System-wide equivalent, which is empty on my Mac, would be here:
/Library/Spelling/
Aside from this, you will find packaged dictionaries here:
/Library/Dictionaries/
~/Library/Dictionaries/

If it were me, I would just add the word to my personal dictionary. Although your question was how to edit the system dictionary, I note that on my system (10.8.2 Australian English) both your words are accepted.
But Microsoft Word 2011 (with Australian English) puts red lines under expresso and is happy with espresso.
It appears your example may be one of those where there is different usage in different parts of the world and in different software packages.