That is a question best asked in the following manner: "What would it require to [manipulate data through a fiber line], and are there existing technologies/methods that can meet those requirements?"
Granted, all of this is done in the shadow of the assumption we have a full index of known phenomena. If there had been a discovery that allowed photons to be remotely tampered with using fields, for example, we assume that it would be public knowledge. If someone did discover such a thing and a state or other similar interest had the power to monopolize and conceal the understanding such a phenomena existed, then any of our assessments on what is and is not possible become void because someone has something akin to magic that can defeat our understanding of how things work.
To give you my own simple assessment - the answer is that there are a set of situations and circumstances which could allow you to do what you describe. It depends on how the fiber is used and what the people who are sitting on each end will do when hit with the unexpected. If you are talking about a fiber that is regularly not in communications use, then the job is easier… Provided you can handle photonics work. If it is a line that is in constant communication and alarms will be raised if there was a temporary interruption - then it requires humint to pull off.
Other techniques to monkey with the photons without cutting the line would be generally within the realm of magic. There are some theories where we could if we were granted a miracle device or two that could produce a particle or field we otherwise can't produce. There is also the possibility there is something more simple could be exploited, such as in the reflection of the waves within the reflective layer - the absorption and re-emission. But this is merely my own speculation that this would be the place to start "hacking reality" - and it is also not always going to be consistent.