Possible to change NNID associated with extdata in order to avoid deletion?

Since Pretendo relies on creating an alternative NNID for your 3DS, some titles (such as Mario vs Donkey Kong Tipping Stars) will detect that you are not using your original NNID and force you to delete the game’s extra data. I was wondering if it’s possible to migrate extdata to your Pretendo ID such that this can be avoided.

That is a good question - unfortunately, I don’t know. In theory, it might be possible through save editing.