You can already import invalid pairs in Hexawise today. Here are the steps:
1. Export your plan with at least 1 invalid pair (this ensures you get an invalid pair sheet in the Excel export)
2. Fill out the rest of the invalid pair sheet in the Excel file (make sure you save it as Excel XML.
3. Create a new plan and import the updated Excel file. The invalid pairs you entered on the invalid pairs sheet will be imported as well.