New Post: Add Button to Content?

Hi!

If you mean something like this:

<button>Click me!</button>

Then you can use the same approach used when defining <h1> etc.

Add to your common.xml something like this:

<format id="button" label="Button" image="" notes="Button" block="button">
  <select />
</format>

This will add a "Button" item to the Style selector list. Then you'll need some block element on a page, e.g.

Click me!

You should place a cursor on it and apply the "Button" form the list. This will wrap the text or other element with <button/> tags.

Since the button is always clickable and not that selectable, you might have hard time selecting it to change, say, back to "Paragraph".
You might need to switch to the HTML editor and remove those <button/> tags around the text or other elements manually.