I understand that you're looking for a way to create vertical text within a table's cell. Unfortunately, this is not supported with the Jive rich text editor (RTE), as the RTE only supports left to right and in new versions, right to left. I don't see that vertical directional text is on the list of existing or planned improvements. If this is an important feature you'd like to see added in the future, I'd recommend submitting an idea in the Ideas for Jive space to help capture your use case and community sentiment for our Products team.
Let me know if you had any other questions on this.