There is no such feature, to best of my knowledge. The whole control is either read-only or not. RTF is a very old standard, pretty much obsolete, I'm not sure you want to mess with it at all. You have to think of something else. If you explain your ultimate goal, I'm almost sure you can get some good advice.
[EDIT]
So, after some (quite vague) clarification, I can suggest some alternative: this brilliant CodeProject work:
A Professional HTML Renderer You Will Use[
^].
This is an HTML rendered or forms. It is also based on RTF, but all RTF trickery is hidden from you in the component library. I tested it and find the features wonderful, especially special URI schema for resources.
Now, you understand that you can have part of HTML editable. The simplest case is: you can scatter text input element in the text (you can even style them to make them seamless). Those element will flow with the text, behaving pretty much like text element.
However, I would advice you to think about your design ideas more. I'm not sure if you are inventing a convenient and natural way of entering data. Just think more.
—SA
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