Currently, the Kendo UI Toolbar supports adding icons to items by setting the "icon" property as documented here. However, only Kendo's own icons are supported.
In other widgets (like the Grid), adding custom icons (to a toolbar!) is already possible by specifiying an icon class.
However, if you wanted a third-party icon (or one of your own) directly to a standalone Toolbar, you would need to create a whole new template from scratch or replace the DOM element after rendering or use some hacky approach like demonstrated in this DOJO. While the first one means a lot of work, the latter ones are both not very stable.
Therefore, I propose to extend the icon support for Toolbars in general. I can think of two different solutions: