If we fork a repo (such as testFork repo) from the original repo (such as test repo), it's only available to create pull request from fork repo (testFork) to the original repo (test):

And it's not available to create pull request from original repo (test) to fork repo (testFork).

And it will be covenient if bitbucket can support the feature to create pull request to merge changes from original repo to fork repo.