How to override PB branch merge proposal?

When merging a branch I can see the view of both source and target and what’s been taken from each. How can I override this proposal? Concretely, in the proposal I see that a piece of the pipeline is going to be deleted after the merge. However, this piece of the pipeline in target branch is independent of the changes in the source branch and I want to keep it. I don’t see any place where I can mark these transformations not to be deleted and kept as already in the target branch.