As far as I know there's no command (API or scripting) to add an object (text, quad or other type) to a textobject, but the guys from Ross will surely correct me if I'm wrong. As you correctly stated you can only add a string to an existing textobject (again, as far as I know).
It is however possible to achieve the effect you want by aligning separate objects in your script. You can get the dimensions of a textobject and use that info together with the position data of the textobject to position your quad so the quad is next to your textobject. And when you put a different text in the textobject, the dimensions will change and the quad will be repositioned automaticly so it is next to your text again.
#XPression