Is it possible to add columns or swimlanes to an existing Kanban board?

Yes, you can edit your workflow at any time.

While on a board, please go to Settings - Board Editor. You can add columns and swimlanes from there with the green +add column and +add swimlane buttons.
You may also change the order in which the columns appear on the board, remove or rename them. The look of any board can be altered at any time.

Please note, board Settings are not accessible to all account users. Team members, who can access Settings are those, that were given at least a project manager privilege, or were made a board owner.