Create toolbar in bottom

If you make that cell as a custom component, you can manipulate its View

  const comps = editor.DomComponents
  comps.addType('cell', {
    isComponent(el) {...},
    model: {...},
    view: {
      events: {
        'click #extra-toolbar': 'doSomething'
      },
      onRender() {
        const el = this.$el

        el.append(`<button id="extra-toolbar">Extra button</button>`)
      },
      doSomething() {
        alert('toolbar clicked')
      }
    }
  })

PS: this new syntax requires version 0.14.50+

not only in the cell, but for all components

Of course, you can apply the same technique to any component.