RichTextBox help

Hi everyone,
I have a question... I want to add a Rectangle/Ellipse/Button (different
control classes, other than text) to a RichTextBox. In XAML, it is done like
this:
<RichTextBox>
<FlowDocument>
<Paragraph>
Some Text!!!
<Button>This is a button</Button>
</Paragraph>
</FlowDocument>
</RichTextBox>

But how do you insert it using code? ie. dynamically.

I'm pretty sure it has something to do with the RichTextBox.Document class,
but I cannot find any information on it.

Can anyone help?

Thanks in advance,
Tim.