inserting text, custom tags into a RichTextBox

I am trying to insert a custom tag that I wrote by subclassing the Run class into a RichTextBox's FlowDocument, but cant seem to find out how to do it. Anyone know the anwer this one?

Also, how fo you insert text into a RichTextBox? Can it be done, or does the contents of a RichTextBox need to be reassigned everytime you wish to add something to it?