This cannot be done.
- You could apply your “fixes” when you actually analyze the data (e.g., define some alias “corrected_branch = original_branch - corerection_value” and then use the “corrected_branch” in the analysis).
- You could “copy” your tree to another file, modifying its entries on the fly.
- You could create a “friend tree” with new branches.
- You could add new branches to the existing tree.