Long answer short: No. Not unless you have permission to edit that person's fork. You can, however, pull in the person's commits and work on top of them as shown below. Note that this will not modify the PR, but only enable you to keep the person's commits.
You could create a new branch. Then point a remote to the branch on which A submitted the PR, pull in changes from A's branch and then work on those changes.
Steps:
Assume A's fork lies at https://github.com/repo/A.git
and A's branch name is <A's_branch_name>
.
- Create a new branch on Github (say
aBranch
).
- Run a
git pull
to pull in the branch.
- Change to the new branch using
git checkout aBranch
.
- Create a remote pointing to A's branch using
git remote add aBranchRemote https://github.com/repo/A.git
.
- Pull in A's changes with a
git merge aBranchRemote <A's_branch_name>
PS: I've not tested these steps, but it should work fine. I faced a very similar situation yesterday, and this worked.