There is no way to install an alternate default English dictionary, but there are dictionaries for other languages available. But it sounds as though that's not what you're looking for. One work around, if you're using technical or business terms, is a suggestion made by another Member. He noted that the iPad never flags as spelling errors, the names of contacts in the iPad's native Contacts app. So he suggested - and it works just fine - put technical and business terms into your Contacts list as 'spoof' contacts.