Use `git rebase`. Specifically:

* Use `git stash` to store the changes you want to add.
* Use `git rebase -i HEAD~10` (or however many commits back you want to see).
* Mark the commit in question `(a0865...)` for edit by changing the word `pick` at the start of the line into `edit`. Don't delete the other lines as that would delete the commits.
* Save the rebase file, and git will drop back to the shell and wait for you to fix that commit.
* Pop the stash by using `git stash pop`
* Add your file with `git add <file>`.
* Amend the commit with `git commit --amend --no-edit`.
* Do a `git rebase --continue` which will rewrite the rest of your commits against the new one.
* Repeat from step 2 onwards if you have marked more than one commit for edit.