No, the field would not be removed but there is a small chance it could clash with a future update. For example if you named the field "abc" then Q2A added a new field "abc" in the next version, upgrading Q2A would fail.
However, there should never be a need to do this. With plugins you can add your own tables to do things.
Edit: and for data relating to posts specifically, there is already a table to add that stuff. See the functions in qa-include/db/metas.php for more info.
