That's not rich-text; rich-text is more like the RichTextBox available in VB
.NET windows forms, where you can color information and perform other more "flavorful" operations with it.
If you want to dynamically add text, you can do that, sure.
The best approach is to dynamically create the ASP.NET server controls in the code-behind (or in the script for in-line approach), and add them to a collection control, such as a panel. Or if you want to do the HTML in a string, you could add the text to a literalcontrol, and add the literalcontrol to a collection, as such: